Microtubule targeting of substrate contacts promotes their relaxation and dissociation.

We recently showed that substrate contact sites in living fibroblasts are specifically targeted by microtubules (Kaverina, I., K. Rottner, and J.V. Small. 1998. J. Cell Biol. 142:181-190). Evidence is now provided that microtubule contact targeting plays a role in the modulation of substrate contact dynamics. The results are derived from spreading and polarized goldfish fibroblasts in which microtubules and contact sites were simultaneously visualized using proteins conjugated with Cy-3, rhodamine, or green fluorescent protein.For cells allowed to spread in the presence of nocodazole the turnover of contacts was retarded, as compared with controls and adhesions that were retained under the cell body were dissociated after microtubule reassembly. In polarized cells, small focal complexes were found at the protruding cell front and larger adhesions, corresponding to focal adhesions, at the retracting flanks and rear. At retracting edges, multiple microtubule contact targeting preceded contact release and cell edge retraction. The same effect could be observed in spread cells, in which microtubules were allowed to reassemble after local disassembly by the application of nocodazole to one cell edge. At the protruding front of polarized cells, focal complexes were also targeted and as a result remained either unchanged in size or, more rarely, were disassembled. Conversely, when contact targeting at the cell front was prevented by freezing microtubule growth with 20 nM taxol and protrusion stimulated by the injection of constitutively active Rac, peripheral focal complexes became abnormally enlarged. We further found that the local application of inhibitors of myosin contractility to cell edges bearing focal adhesions induced the same contact dissociation and edge retraction as observed after microtubule targeting.Our data are consistent with a mechanism whereby microtubules deliver localized doses of relaxing signals to contact sites to retard or reverse their development. We propose that it is via this route that microtubules exert their well-established control on cell polarity.     

Effect of muscle and non-muscle tropomyosins in reconstituted skeletal muscle actomyosin

Smooth and non-muscle tropomyosins were found to produce a 2-3-fold Ca-insensitive stimulation of the ATPase activity of reconstituted skeletal muscles actomyosin at normal MgATP concentrations and physiological ratios of myosin to actin. Under the same conditions skeletal muscles tropomyosin had no effect. Similar effects of these three tropomyosins were observed for the low myosin/F-actin ratios necessary for kinetic measurements. Since it could be established that this actomyosin system, with or without tropomyosin, obeyed Michaelian kinetics, the tropomyosin effects could be interpreted in terms of their influence on maximal turnover (V) or on the affinity of myosin for actin (Kapp). Accordingly, gizzard tropomyosin had practically no effect on the affinity and reduced only slightly the value of V, compared to pure actin. In contrast to gizzard tropomyosin, brain tropomyosin produced an approximately twofold increase in both Kapp and V; i.e. it increased the turnover rate but decreased the affinity. It is apparent from the data that brain tropomyosin acts as an uncompetitive activator with respect to pure actin, while having the same V as the actin plus gizzard tropomyosin complex. Further studies on these tropomyosins show that only skeletal and smooth muscle tropomyosin have similar functional properties with respect to troponin inhibition and the activation of the ATPase at low ATP concentrations. It is suggested that the noted increases in V by tropomyosin are caused by the acceleration of the dissociation of the myosin head from actin at the end point of the cross bridge movement.     

Isolation and sequence of a tropomyosin-binding fragment of turkey gizzard calponin

Limited chymotryptic cleavage of turkey gizzard calponin yields a 13 kDa fragment which could be purified by its ability to bind to Sepharose-immobilized tropomyosin. This 13 kD polypeptide is shown to be derived from a 22 kDa fragment. Complete amino acid sequence analysis of the 13 kD and 22 kD fragments reveals high homology with the formerly characterized smooth muscle-specific protein SM22 alpha (Pearlstone, J.R., Weber, M., Lees-Miller, J.P., Carpenter, M.R. and Smillie L.B., 1987, J. Biol. Chem. 262, 5985-5991) and the product of gene mp20 of Drosophila (Ayme-Southqate, A., Lasko, P., French, C, and Pardue, M.L. [(1989) J. Cell Biol. 108, 521-531]. Futhermore we recognize sequence elements of a putative actin-binding domain of alpha-actinin, the calpactin I or p 36 sequence, and a consensus motif present in the repeats of the gene product of the candidate unc-87 gene of C. elegans (S.D. Goetinck and R.H. Waterston, personal communication).     

Identification of BPESC1, a Novel Gene Disrupted by a Balanced Chromosomal Translocation, t(3;4)(q23;p15.2), in a Patient with BPES

The blepharophimosis syndrome (BPES) is a rare genetic disorder characterized by blepharophimosis, ptosis, epicanthus inversus, and telecanthus. In type I, BPES is associated with female infertility, while in type II, the eyelid defect occurs by itself. The BPES syndrome has been mapped to 3q23. Previously, we constructed a YAC-, PAC-, and cosmid-based physical map surrounding the 3q23 translocation breakpoint of a t(3;4)(q23;p15.2) BPES patient, containing a 110-kb PAC (169-C 10) and a 43-kb cosmid (11-L 10) spanning the breakpoint. In this report, we present the identification of BPESC1 (BPES candidate 1), a novel candidate gene that is disrupted by the translocation on chromosome 3. Cloning of the cDNA has been performed starting from a testis-specific EST, AI032396, found in cosmid 11-L 10. The cDNA sequence of BPESC1 is 3518 bp in size and contains an open reading frame of 351 bp. No significant similarities with known proteins have been found in the sequence databases. BPESC1 contains three exons and spans a genomic fragment of 17.5 kb. Expression of BPESC1 was observed in adult testis tissue. We performed mutation analysis in 28 unrelated familial and sporadic BPES patients, but, apart from the disruption by the translocation, found no other disease-causing mutations. These data make it unlikely that BPESC1 plays a major role in the pathogenesis of BPES.     

Repair of 3-methyladenine and 7-methylguanine in nuclear DNA of Chlamydomonas: Requirement for protein synthesis

The removal of 3-methyladenine and 7-methylguanine from nuclear DNA was determined following exposure of Chlamydomonas reinhardi to methyl methanesulfonate (MMS). The amount of 3-methyladenine in DNA was determined using an extract from Micrococcus luteus that has a 3-methyladenine-DNA glycosylase. The amount of 7-methylguanine was estimated by heating the DNA for 30 min at 70° followed by alkaline hydrolysis of the resulting apurinic sites. The molecular weight of the DNA was determined using alkaline sucrose gradients. The 3-methyladenine is removed with a half-life of 2–3 h whereas the 7-methylaguanine is removed with a half-life of 10–12 h. The rate of removal of the 7-methylguanine is more than an order of magnitude faster than the estimated non-enzymatic hydrolysis rate indicating the probability of enzymatic repair. Addition of cycloheximide immediately after MMS treatment inhibits the removal of 3-methyladenine and 7-methylguanine from DNA. If cycloheximide is added 1.5 h after treatment with MMS, there is much less inhibition of the removal of 3-methyladenine. These results are interpreted to mean that MMS induces the synthesis of 1 or more proteins that are required for the repair of 3-methyladenine from Chlamydomonas DNA.