Dissection of inhibitory Smad proteins: both N- and C-terminal domains are necessary for full activities of Xenopus Smad6 and Smad7

Smad6 and Smad7 comprise a subclass of vertebrate Smads that antagonize, rather than transduce, TGF-β family signaling. These Anti-Smads can block BMP signaling, as evidenced by their ability to induce a secondary dorsal axis when misexpressed ventrally in Xenopus embryos. Smad7 inhibits additional TGF-β related pathways, and causes spina bifida when misexpressed dorsally. We have performed structure-function analyses to identify domains of Anti-Smads that are responsible for their shared and unique activities. We find that the C-terminal domain of Smad7 displays strong axis inducing activity but cannot induce spina bifida. The isolated N-terminal domain of Smad7 is inactive but restores the ability of the C-terminus to cause spina bifida when the two are co-expressed. By contrast, the N- and C-terminal domains of Smad6 have weak axis inducing activity when expressed individually, but show full activity when co-expressed. Chimeric analysis demonstrates that the C-terminal domain of Smad7, but not Smad6, can induce spina bifida when fused to the N-terminal domain of either Smad6 or Smad7. Thus, although the C-terminal domain is the primary determinant of the intrinsic activity of Xenopus Anti-Smads, the N-terminal domain is essential for full activity, is interchangeable between Smad6 and 7, and can function in trans.     

Dissection of inhibitory Smad proteins: both N- and C-terminal domains are necessary for full activities of Xenopus Smad6 and Smad7

Smad6 and Smad7 comprise a subclass of vertebrate Smads that antagonize, rather than transduce, TGF-β family signaling. These Anti-Smads can block BMP signaling, as evidenced by their ability to induce a secondary dorsal axis when misexpressed ventrally in Xenopus embryos. Smad7 inhibits additional TGF-β related pathways, and causes spina bifida when misexpressed dorsally. We have performed structure-function analyses to identify domains of Anti-Smads that are responsible for their shared and unique activities. We find that the C-terminal domain of Smad7 displays strong axis inducing activity but cannot induce spina bifida. The isolated N-terminal domain of Smad7 is inactive but restores the ability of the C-terminus to cause spina bifida when the two are co-expressed. By contrast, the N- and C-terminal domains of Smad6 have weak axis inducing activity when expressed individually, but show full activity when co-expressed. Chimeric analysis demonstrates that the C-terminal domain of Smad7, but not Smad6, can induce spina bifida when fused to the N-terminal domain of either Smad6 or Smad7. Thus, although the C-terminal domain is the primary determinant of the intrinsic activity of Xenopus Anti-Smads, the N-terminal domain is essential for full activity, is interchangeable between Smad6 and 7, and can function in trans.     

The two homologous domains of human angiotensin I-converting enzyme are both catalytically active.

Molecular cloning of human endothelial angiotensin I-converting enzyme (kininase II; EC 3.4.15.1) (ACE) has recently shown that the enzyme contains two large homologous domains (called here the N and C domains), each bearing a putative active site, identified by sequence comparisons with the active sites of other zinc metallopeptidases. However, the previous experiments with zinc or competitive ACE inhibitors suggested a single active site in ACE. To establish whether both domains of ACE are enzymatically active, a series of ACE mutants, each containing only one intact domain, were constructed by deletion or point mutations of putative critical residues of the other domain, and expressed in heterologous Chinese hamster ovary cells. Both domains are enzymatically active and cleave the C-terminal dipeptide of hippuryl-His-Leu or angiotensin I. Moreover, both domains have an absolute zinc requirement for activity, are activated by chloride and are sensitive to competitive ACE inhibitors, and appear to function independently. However, the two domains display different catalytic constants and different patterns of chloride activation. At high chloride concentrations, the C domain hydrolyzes the two substrates tested faster than does the N domain. His-361,365 and His-959,963 are established as essential residues in the N and C domains, respectively, most likely involved in zinc binding, and Glu-362 in the N domain and Glu-960 in the C domain are essential catalytic residues. These observations provide strong evidence that ACE possesses two independent catalytic domains and suggest that they may have different functions.     

The 10 C-terminal residues of HTLV-I protease are not necessary for enzymatic activity

Sequence alignment of human T-lymphotropic virus type I (HTLV-I) protease and other retroviral proteases reveals that the leukemia virus proteases contain residues at the C-terminus that are absent in the other proteases. We have prepared a mutant of HTLV-I protease that does not contain the 10 C-terminal residues and demonstrated that the catalytic efficiency of cleavage of a peptide substrate is unaffected.     

Structural domains of the poliovirus polyprotein are major determinants for proteolytic cleavage at Gln-Gly pairs

The processing of poliovirus precursor polypeptides provides a valuable system in which to study the recognition and interaction of a proteolytic enzyme with its substrates. Processing of the poliovirus polyprotein includes cleavage between 9 of 13 available glutamineglycine (Q-G) pairs by the activity of a virally encoded proteinase, 3C. In this study, we assess the importance of primary, secondary, and tertiary structural determinants in the cleavage at two Q-G pairs in the capsid protein precursor, P1. Employing site-directed mutagenesis of cDNA copies of poliovirus RNA, we have made specific alterations in regions of the P1 capsid precursor and have assayed the effect of these alterations on proteinase cleavage at the two Q-G pairs. We have also introduced additional Q-G pairs into P1 and demonstrated that the proteinase can recognize some of the inserted Q-G pairs as cleavage sites. By correlating the predicted three-dimensional structures and the processing phenotypes of several altered P1 precursors, we are able to rank the importance of determinants required for P1 processing. While a Q-G pair appears to be the primary determinant in proteinase recognition, the tertiary location of a Q-G pair in the precursor either allows or prevents processing at that pair. Our results also suggest that the proper folding of at least two of the three P1 beta-barrel structures is required for efficient proteinase cleavage at Q-G pairs.     

Spleen necrosis virus gag polyprotein is necessary for particle assembly and release but not for proteolytic processing.

The nature of spleen necrosis virus pol gene expression and the role of gag and gag-pol polyproteins in virion assembly was investigated. The DNA sequence of the gag-pol junction revealed that the two genes occupy the same open reading frame but are separated by an in-frame amber stop codon. Biochemical analysis of gag-pol translational readthrough in vitro and in Escherichia coli suggests that, in a manner similar to that in other mammalian type C retroviruses, amber stop codon suppression is required for pol gene expression. Removal of the gag stop codon had little or no effect on synthesis or cleavage of the polyprotein but interrupted particle assembly. This block could be overcome by complementation with wild-type gag protein.     

Two conserved domains in the NGF propeptide are necessary and sufficient for the biosynthesis of correctly processed and biologically active NGF.

The three members of the neurotrophin family (NGF, BDNF and NT-3) are synthesized as large precursor proteins which undergo proteolytic processing to yield biologically active, mature neurotrophic factors. We have used in vitro mutagenesis to examine the pro-region in the NGF precursor protein as a first step towards a general understanding of the role of propeptides in the biosynthesis of neurotrophins. Our results demonstrate that only two small domains within the NGF propeptide are required for the expression and secretion of properly processed and biologically active, recombinant mouse NGF in COS-7 cells. Domain I plays an important role in the expression of active NGF while domain II is involved in proteolytic processing. Both domains are partially conserved between the propeptides of NGF proteins isolated from different species as well as BDNF and NT-3.     

The interplay of the N- and C-terminal domains of MCAK control microtubule depolymerization activity and spindle assembly

Spindle assembly and accurate chromosome segregation require the proper regulation of microtubule dynamics. MCAK, a Kinesin-13, catalytically depolymerizes microtubules, regulates physiological microtubule dynamics, and is the major catastrophe factor in egg extracts. Purified GFP-tagged MCAK domain mutants were assayed to address how the different MCAK domains contribute to in vitro microtubule depolymerization activity and physiological spindle assembly activity in egg extracts. Our biochemical results demonstrate that both the neck and the C-terminal domain are necessary for robust in vitro microtubule depolymerization activity. In particular, the neck is essential for microtubule end binding, and the C-terminal domain is essential for tight microtubule binding in the presence of excess tubulin heterodimer. Our physiological results illustrate that the N-terminal domain is essential for regulating microtubule dynamics, stimulating spindle bipolarity, and kinetochore targeting; whereas the C-terminal domain is necessary for robust microtubule depolymerization activity, limiting spindle bipolarity, and enhancing kinetochore targeting. Unexpectedly, robust MCAK microtubule (MT) depolymerization activity is not needed for sperm-induced spindle assembly. However, high activity is necessary for proper physiological MT dynamics as assayed by Ran-induced aster assembly. We propose that MCAK activity is spatially controlled by an interplay between the N- and C-terminal domains during spindle assembly.     

The cysteine-rich and C-terminal domains of dystrophin are not required for normal costameric localization in the mouse

Dystrophin has a modular structure and is believed to be critical for muscle cell cytoarchitecture by linking the cytoskeleton to the extracellular matrix. The N-terminus binds to actin and two domains at the C-terminus, the cysteine-rich and C-terminal domains, are associated with the sarcolemma indirectly via the dystroglycan complex. We have generated a mutation in mouse embryonic stem (ES) cells which serves to delete the cysteine-rich and C-terminal domains to address directly their role. We show that these two domains are not necessary for normal costameric organization at the sarcolemma in myotubes derived from the mutant cell line. Furthermore sarcolemmal localization is also apparent in mouse chimaeric musclein vivo.     

Concerted activities of the RNA recognition and the glycine-rich C-terminal domains of nucleolin are required for efficient complex formation with pre-ribosomal RNA.

Nucleolin is an abundant nucleolar protein which is involved in the early stages of ribosome assembly. The central 40-kDa domain of nucleolin comprises four RNA recognition motifs (RRM) which are presumed to be involved in specific interactions with pre-rRNA. In order to examine in detail the role of this central domain and the contribution of the N-terminal and C-terminal domains of nucleolin to RNA binding, we have used an Escherichia coli expression system to synthezise polypeptides corresponding to various combinations of the three domains and their subdomains. By means of an in-vitro binding assay and a synthetic RNA corresponding to a specific recognition site in pre-rRNA we have been able to demonstrate conclusively that the central 40-kDa domain is indeed responsible for the specificity of RNA recognition and that the N-terminal domain can be removed without affecting RNA binding. Most interestingly, it appears that the C-terminal 10-kDa domain, which is rich in glycine and arginine residues, is essential for efficient binding of nucleolin to RNA, but does not itself contribute to the specificity of the interaction. Circular dichroic spectroscopic probing of the RNA component shows that the C-terminal domain significantly modifies the RNA-binding properties of the central RRM core. Finally, infrared spectroscopic studies reveal that the central 40-kDa domain is structured in alpha helices and beta sheets and that the interaction with the specific pre-rRNA site induces subtle changes in the beta sheet conformation.     

The N-terminal and transmembrane domains of Commissureless are necessary for its function and trafficking within neurons

Commissureless (Comm) is a novel transmembrane molecule necessary both for commissural axons to cross the midline of the Drosophila central nervous system and normal synaptogenesis. Comm is able to reduce cell surface levels of Roundabout (Robo), a receptor for the midline repellent Slit, on commissural axons and unknown inhibitors of synaptogenesis expressed on muscle cells. Comm is expressed dynamically and is found at the cell surface and within intracellular vesicles. Comm can bind Robo and when the proteins are co-expressed Robo is found co-localised with Comm intracellularly. Here we show that the ability of Comm to localise intracellularly and hence regulate Robo surface levels requires sequences in both the N-terminal and transmembrane domains. We also show that Comm can dimerise via its N-terminal domain. Furthermore, absence of the Comm N-terminal and transmembrane regions results in the protein being restricted to the neuron soma.     

Two C-terminal cysteines are necessary for proper folding of the peptidase neprilysin/CD10

Neprilysin (NEP) consists of 749 amino acids with two conserved cysteines (734, 746) and a putative CAAX motif (residues 746-749, CRVW) at the C-terminus. To investigate the role of the C-terminal conserved cysteine residues, three NEP mutants (C734S, C746S, and double mutant C734S/C746S) were constructed by use of site-directed mutagenesis. Western blot analysis of lysates of transfected cells revealed the presence of three NEP forms in wild type and mutants with a different glycosylation pattern. Point mutations of C734 as well as C746 by serine dramatically diminished the plasma membrane association of NEP as detected by flow cytometry and laser scanning microscopy. Endoprotease enzyme activity was slightly diminished in the C746S-NEP variant and was not detectable in the C734S-form of NEP suggesting a pivotal role of the C734 in the proper folding of the enzyme. Prenylation of NEP was not detected in an in vivo assay.     

The C-terminal N-glycosylation sites of the human alpha1,3/4-fucosyltransferase III, -V, and -VI (hFucTIII, -V, adn -VI) are necessary for the expression of full enzyme activity

The alpha1,3/4-fucosyltransferases are involved in the synthesis of fucosylated cell surface glycoconjugates. Human alpha1,3/4-fucosyltransferase III, -V, and -VI (hFucTIII, -V, and -VI) contain two conserved C-terminal N-glycosylation sites (hFucTIII: Asn154 and Asn185; hFucTV: Asn167 and Asn198; and hFucTVI: Asn153 and Asn184). In the present study, we have analyzed the functional role of these potential N-glycosylation sites, laying the main emphasis on the sites in hFucTIII. Tunicamycin treatment completely abolished hFucTIII enzyme activity while castanospermine treatment diminished hFucTIII enzyme activity to approximately 40% of the activity of the native enzyme. To further analyze the role of the conserved N-glycosylation sites in hFucTIII, -V, and -VI, we made a series of mutant genomic DNAs in which the asparagine residues in the potential C-terminal N-glycosylation sites were replaced by glutamine. Subsequently, the hFucTIII, -V, and -VI wild type and the mutants were expressed in COS-7 cells. All the mutants exhibited lower enzyme activity than the wild type and elimination of individual sites had different effects on the activity. The mutations did not affect the protein level of the mutants in the cells, but reduced the molecular mass as predicted. Kinetic analysis of hFucTIII revealed that lack of glycosylation at Asn185 did not change the Km values for the oligosaccharide acceptor and the nucleotide sugar donor. The present study demonstrates that hFucTIII, -V, and -VI require N-glycosylation at the two conserved C-terminal N-glycosylation sites for expression of full enzyme activity.     

Calcium-induced conformational changes in C-terminal tail of polycystin-2 are necessary for channel gating

Polycystin-2 (PC2) is a Ca²⁺-permeable transient receptor potential channel activated and regulated by changes in cytoplasmic Ca²⁺. PC2 mutations are responsible for ∼15% of autosomal dominant polycystic kidney disease. Although the C-terminal cytoplasmic tail of PC2 has been shown to contain a Ca²⁺-binding EF-hand domain, the molecular basis of PC2 channel gating by Ca²⁺ remains unknown. We propose that the PC2 EF-hand is a Ca²⁺ sensor required for channel gating. Consistent with this, Ca²⁺ binding causes a dramatic decrease in the radius of gyration (R_g) of the PC2 EF-hand by small angle x-ray scattering and significant conformational changes by NMR. Furthermore, increasing Ca²⁺ concentrations cause the C-terminal cytoplasmic tail to transition from a mixture of extended oligomers to a single compact dimer by analytical ultracentrifugation, coupled with a >30 Å decrease in maximum interatomic distance (D_max) by small angle x-ray scattering. Mutant PC2 channels unable to bind Ca²⁺ via the EF-hand are inactive in single-channel planar lipid bilayers and inhibit Ca²⁺ release from ER stores upon overexpression in cells, suggesting dominant negative properties. Our results support a model where PC2 channels are gated by discrete conformational changes in the C-terminal cytoplasmic tail in response to changes in cytoplasmic Ca²⁺ levels. These properties of PC2 are lost in autosomal dominant polycystic kidney disease, emphasizing the importance of PC2 to kidney cell function. We speculate that PC2 and the Ca²⁺-dependent transient receptor potential channels in general are regulated by similar conformational changes in their cytoplasmic domains that are propagated to the channel pore.     

C-terminal domains implicated in the functional surface expression of potassium channels

A short C-terminal domain is required for correct tetrameric assembly in some potassium channels. Here, we show that this domain forms a coiled coil that determines not only the stability but also the selectivity of the multimerization. Synthetic peptides comprising the sequence of this domain in Eag1 and other channels are able to form highly stable tetrameric coiled coils and display selective heteromultimeric interactions. We show that loss of function caused by disruption of this domain in Herg1 can be rescued by introducing the equivalent domain from Eag1, and that this chimeric protein can form heteromultimers with Eag1 while wild-type Erg1 cannot. Additionally, a short endoplasmic reticulum retention sequence closely preceding the coiled coil plays a crucial role for surface expression. Both domains appear to co-operate to form fully functional channels on the cell surface and are a frequent finding in ion channels. Many pathological phenotypes may be attributed to mutations affecting one or both domains.     

Central and C-terminal domains of heterotrimeric G protein gamma subunits differentially influence the signaling necessary for primordial germ cell migration

Heterotrimeric G protein signaling is involved in many pathways essential to development including those controlling cell migration, proliferation, differentiation and apoptosis. One key developmental event known to rely on proper heterotrimeric G protein signaling is primordial germ cell (PGC) migration. We previously developed an in vivo PGC migration assay that identified differences in the signaling capacity of G protein gamma subunits. In this study we developed Gγ subunit chimeras to determine the regions of Gγ isoforms that are responsible for these differences. The central section of the Gγ subunit was found to be necessary for the ability of a Gγ subunit to mediate signaling involved in PGC migration. Residues found in the carboxy-terminal segment of Gγ transducin (gngt1) were found to be responsible for the ability of this subunit to disrupt PGC migration. The type of prenylation did not affect the ability of a Gγ subunit to reverse prenylation-deficient-Gγ-induced PGC migration defects. However, a version of gng2, engineered to be farnesylated instead of geranylgeranylated, still lacks the ability to reverse PGC migration defects known to result from treatment of zebrafish with geranylgeranyl transferase inhibitors (GGTI), supporting the notion that Gγ subunits are one of several protein targets that need to be geranylgeranylated to orchestrate the proper long-range migration of PGCs.     

C-terminal and heparin-binding domains of collagenic tail subunit are both essential for anchoring acetylcholinesterase at the synapse

The collagen-tailed form of acetylcholinesterase (A(12)-AChE) appears to be localized at the neuromuscular junction in association with the transmembrane dystroglycan complex through binding of its collagenic tail (ColQ) to the proteoglycan perlecan. The heparan sulfate binding domains (HSBD) of ColQ are thought to be involved in anchoring ColQ to the synaptic basal lamina. The C-terminal domain (CTD) of ColQ is also likely involved, but there has been no direct evidence. Mutations in COLQ cause endplate AChE deficiency in humans. Nine previously reported and three novel mutations are in CTD of ColQ, and most CTD mutations do not abrogate formation of A(12)-AChE in transfected COS cells. Patient endplates, however, are devoid of AChE, suggesting that CTD mutations affect anchoring of ColQ to the synaptic basal lamina. Based on our observations that purified AChE can be transplanted to the heterologous frog neuromuscular junction, we tested insertion competence of nine naturally occurring CTD mutants and two artificial HSBD mutants. Wild-type human A(12)-AChE inserted into the frog neuromuscular junction, whereas six CTD mutants and two HSBD mutants did not. Our studies establish that the CTD mutations indeed compromise anchoring of ColQ and that both HSBD and CTD are essential for anchoring ColQ to the synaptic basal lamina.     

Sequences within both the N- and C-terminal domains of phytochrome A are required for PFR ubiquitination and degradation

Photoconversion of the plant photoreceptor phytochrome A (phyA) from its inactive Pr form to its biologically active Pfr from initiates its rapid proteolysis. Previous kinetic and biochemical studies implicated a role for the ubiquitin/26S proteasome pathway in this breakdown and suggested that multiple domains within the chromoprotein are involved. To further resolve the essential residues, we constructed a series of mutant PHY genes in vitro and analyzed the Pfr-specific degradation of the resulting photoreceptors expressed in transgenic tobacco. One important site is within the C-terminal half of the polypeptide as its removal stabilizes oat phyA as Pfr. Within this half is a set of conserved lysines that are potentially required for ubiquitin attachment. Substitution of these lysines did not prevent ubiquitination or breakdown of Pfr, suggesting either that they are not the attachment sites or that other lysines can be used in their absence. A small domain just proximal to the C-terminus is essential for the form-dependent breakdown of the holoprotein. Removal of just six amino acids in this domain generated a chromoprotein that was not rapidly degraded as Pfr. Using chimeric photoreceptors generated from potato PHYA and PHYB, we found that the N-terminal half of phyA is also required for Pfr-specific breakdown. Only those chimeras containing the N-terminal sequences from phyA were ubiquitinated and rapidly degraded as Pfr. Taken together, our data demonstrate that, whereas an intact C-terminal domain is essential for phyA degradation, the N-terminal domain is responsible for the selective recognition and ubiquitination of Pfr.