uDENN, DENN, and dDENN: indissociable domains in Rab and MAP kinases signaling pathways

DENN domains are found in a variety of signaling proteins but their exact function remains undefined. Some of the DENN-containing proteins, such as rat Rab3GEP (Rab3 GDP/GTP exchange protein) or mouse Rab6IP1 (Rab6 interacting protein 1) interact with GTPases of the Rab family. Others, such as human MADD (MAP (Mitogen-activated protein) kinase activating protein containing death domain) and human ST5 (Suppressor of tumoreginicity 5) gene products are involved in regulation of MAPKs (Mitogen-activated protein kinases) signaling pathways. Using a combination of profile-based and bidimensional analyses, we show here that DENN domains are much larger than described to date in domain databases, always encircled on both sides by more divergent domains, that we called uDENN and dDENN. These however share conserved amino acids which could play a key role in the DENN functions.     

Functional specificity conferred by the unique plasticity of fully alpha-helical Ras and Rho GAPs

Structural comparisons of the two GTPase activating proteins (GAPs) p120 and p50 in complex with Ras and Rho, respectively, allowed us to decipher the functional role of specific structural features, such as helix alpha8c of p120 and helix A1 of p50, necessary for small GTPase recognition. We identified important residues that may be critical for stabilization of the GAP/GTPase binary complexes. Detection of topohydrophobic positions (positions which are most often occupied by hydrophobic amino acids within a family of protein domains) conserved between the two GAP families led to the characterization of a common flexible four-helix bundle. Altogether, these data are consistent with a rearrangement of several helices around a common core, which strongly supports the assumption that p50 and p120 GAPs derive from a unique fold. Considered as a whole, the remarkable plasticity of GAPs appears to be a means used by nature to accurately confer functional specificity.     

The gamma-subunit of (Na+,K+)-ATPase: a representative example of human single transmembrane protein with a key regulatory role.

The (Na+,K+)-ATPase is a plasma membrane protein complex composed of at least three subunits (alpha,beta,gamma) that couples the exchange of three cytoplasmic Na+ ions with two extracellular K+ ions, to the hydrolysis of one molecule ofATP in most animal cells. The gamma-subunit is a 66 residue membrane protein associated with the active alpha/beta binary complex. It can be considered as an archetype of single transmembrane proteins (type I) which may play a modulatory role upon association with functional membrane partners. This paper highlights similar associations observed with other ATPases such as the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA1/SERCA 2a), but also with Cl- and/or K+ currents, ionic channels (HERG, KCNQ1) and G-protein coupled receptors (adrenomedullin, CGRP and calcitonin) which are of particular interest in the cardiovascular field. Here is reviewed the assessed or suggested regulatory role of a family of small plasma/SR associated membrane proteins including gamma-subunit, phospholemman, Mat 8, KCNE (type 1, 2 and 3), RAMP (type 1, 2 and 3), sarcolipin and phospholamban, mainly found in muscular and vascular tissues. These proteins are critical in controlling important biological processes which derive from specific associations with a binding partner and particular subcellular localizations.