[Differences in phorbol-dependent phosphorylation of regulatory proteins and contraction of phasic and tonic smooth muscle]. / Razlichia v forbolzavisimom fosforilirovanii regliatornykh belkov i sokrashchenii faznykh i tonicheskikh gladkikh myshts.

Smooth muscles are divided into slowly contracting tonic and relatively fast phasic muscles. In both cases Ca2+ is a key mediator of the contractile response. However, the appearance of a tonic component during sphincter or arterial muscle contraction and its absence in contracting visceral smooth muscle is characteristic of their difference. We have found that in chicken tissues phorbol 12,13-dibutyrate (PDBu) induces a sustained contraction in carotid arterial muscle, but provokes no contraction in phasic gizzard smooth muscle. Next we were aimed to find differences in PDBu-induced phosphorylation of the key proteins involved in regulation of smooth muscle contraction, i.e. caldesmon, myosin light chain kinase (MLCK), and the myosin light chain kinase-related protein (KRP, also known as telokin). Two correlative differences were observed. 1. PDBu stimulated phosphorylation of MLCK in tonic smooth muscle and had no effect on the level of MLCK phosphorylation in phasic muscle. Phosphopeptide mapping suggests the involvement of mitogen-activated protein (MAP) kinases in phosphorylation of MLCK in situ. 2. PDBu induced phosphorylation of MAP-kinase sites in caldesmon in both types of smooth muscle, but this phosphorylation had no significant effect on caldesmon functional activity in vitro. For the first time we have shown that in gizzard PDBu also stimulates a yet unknown transitory caldesmon-kinase different from protein kinase, C, Ca2+/calmodulin-dependent kinase II and casein kinase CK2. 3. No significant difference was found in the kinetics of PDBu-dependent phosphorylation of KRP in tonic and phasic smooth muscles. KRP was also demonstrated to be a major phosphoprotein in smooth muscle phosphorylated in vivo at several sites located within its N-terminal sequence. Protein kinases able to phosphorylate these sites were identified in vitro. Among them, MAP-kinase was suggested to phosphorylate a serine residue homologous to that phosphorylated in MLCK. 4. p42erk2 and p38 MAP-kinases were found in phasic and tonic smooth muscles. Both were responsive to PDBu in cultured chicken aortic smooth muscle cells, and their role in phosphorylation of MLCK and low molecular weight isoform of caldesmon was evaluated.

[Functional properties and intracellular localization of high molecular weight isoforms of ligh chain myosin kinase]. / Funktsional'nye svoistva i vnutrikletochnaia lokalizatiia vysokomolekuliarnoi izoformy kinazy legkikh tsepei miozina.

The vertebrate genetic locus, coding for a Ca2+/calmodulin-dependent enzyme myosin light chain kinase (MLCK), the key regulator of smooth muscle contraction and cell motility, reveals a complex organization. Two MLCK isoforms are encoded by the MLCK genetic locus. Recently identified M(r) 210 kDa MLCK contains a sequence of smooth muscle/non-muscle M(r) 108 kDa MLCK and has an additional N-terminal sequence (Watterson et al., 1995. FEBS Lett. 373 : 217). A gene for an independently expressed non-kinase product KRP (telokin) is located within the MLCK gene (Collinge et al., 1992. Mol. Cell. Biol. 12 : 2359). KRP binds to and regulates the structure of myosin filaments (Shirinsky et al., 1993. J. Biol. Chem. 268 : 16578). Here we compared biochemical properties of MLCK-210 and MLCK-108 and studied intracellular localization of MLCK-210. MLCK-210 was isolated from extract of chicken aorta by immunoprecipitation using specific antibody and biochemically analysed in vitro. MLCK-210 phosphorylated myosin regulatory light chain and heavy meromyosin. The Ca(2+)-dependence and specific activity of MLCK-210 were similar to that of MLCK-108 from turkey gizzard. Using sedimentation assay we demonstrated that MLCK-210 as well as MLCK-108 binds to both actin and myosin filaments. MLCK-210 was localized in smooth muscle cell layers of aortic wall and was found to co-localize with microfilaments in cultured aortic smooth muscle cells.