Organization of the genetic locus for chicken myosin light chain kinase is complex: Multiple proteins are encoded and exhibit differential expression and localization

Abstract

We report that the genetic locus that encodes vertebrate smooth muscle and nonmuscle myosin light chain kinase (MLCK) and kinase-related protein (KRP) has a complex arrangement and a complex pattern of expression. Three proteins are encoded by 31 exons that have only one variation, that of the first exon of KRP, and the genomic locus spans approximately 100 kb of DNA. The three proteins can differ in their relative abundance and localization among tissues and with development. MLCK is a calmodulin (CaM) regulated protein kinase that phosphorylates the light chain of myosin II. The chicken has two MLCK isoforms encoded by the MLCK/KRP locus. KRP does not bind CaM and is not a protein kinase. However, KRP binds to and regulates the structure of myosin II. Thus, KRP and MLCK have the same subcellular target, the myosin II molecular motor system. We examined the tissue and cellular localization of KRP and MLCK in the chicken embryo and in adult chicken tissues. We report on the selective localization of KRP and MLCK among and within tissues and on a differential distribution of the proteins between embryonic and adult tissues. The results fill a void in our knowledge about the organization of the MLCK/KRP genetic locus, which appears to be a late evolving regulatory paradigm, and suggest an independent and complex regulation of expression of the gene products from the MLCK/KRP genetic locus that may reflect a basic principle found in other eukaryotic gene clusters that encode functionally linked proteins.

Ca2+Sensitivity of Smooth Muscle and Nonmuscle Myosin II: Modulated by G Proteins, Kinases, and Myosin Phosphatase

“…Long MLCK is prominently expressed in embryonic smooth and nonmuscle cells (181), particularly in endothelium (402), where both forms are present in mature cells, although the long MLCK tends to predominate in cultured endothelial or smooth muscle cells (29), and its expression increases with passage in culture. NH 2 -terminal Tyr phosphorylation of endothelial long smMLCK by p60 src has been suggested to be regulatory, possibly by affecting its subcellular localization (26,27), but it remains to be determined whether it occurs in vivo and affects Ca 2ϩ sensitivity. Localization of enzymes relative to their substrates, such as different localization of, respectively, long and smMLCK (29) and different subcellular Ca 2ϩ domains are likely to result in spatially selective modulation of Ca 2ϩ signaling.…”

“…skMLCK, a somewhat smaller (77-90 kDa) product of a different gene than smMLCK (181), is also regulated by Ca 2ϩ /CaM and can be Ser-phosphorylated by PKA, but this phosphorylation does not affect its Ca 2ϩ /CaM sensitivity (27,89).…”

Drosophila Stretchin-MLCK is a Novel Member of the Titin/Myosin Light Chain Kinase Family

“…The existence of transcripts of various sizes has also been reported for the chicken and human sm/nmMLCK genes (Collinge et al, 1992;Watterson et al, 1995Watterson et al, , 1999. In these cases, a single transcription unit produces three transcripts, the smaller ones being initiated by internal promoters (Birukov et al, 1998, Watterson et al, 1999Smith et al, 1998). If Stretchin-Mlck were to contain internal promoters, we should expect to ®nd (1) characteristic promoter sequences amongst coding exons and expect those promoter sequences to be followed by (2) exons corresponding to the 5 H UTRs od cDNAs and (3) good translation sites.…”

“…Northern blot analyses show that this MLCK-related gene expresses multiple transcripts, whose sizes ranges from 3.2 to greater than 18 kb (Kojima et al, 1996;Tohtong et al, 1997), but the structure of the transcription unit responsible for these transcripts has not been de®ned. In comparison, the chicken and human sm/nmMLCK genes also produce multiple transcripts, ranging from 2.7 to 9 kb in size, which in turn are translated into proteins whose size ranges from 17 to 210 kDa (Collinge et al, 1992;Watterson et al, 1995Watterson et al, , 1999Birukov et al, 1998). This``embedded gene'' concept enables a single transcription unit to produce several transcripts and protein isoforms via internal promoters and alternative translation starts (Watterson et al, 1995(Watterson et al, , 1999Birukov et al, 1998;Smith et al, 1998).…”

Myosin light chain kinase regulates hearing in mice by influencing the F-actin cytoskeleton of outer hair cells and cochleae

“…Short MLCK is expressed in hair cells and is necessary for maintaining membrane stability in IHCs (21). Short MLCK has a catalytic core, a regulatory segment, 3 immunoglobulin-like modules, a fibronectin module, a PEVK repeat-rich region and a 3DFRXXL F-actin binding motif in the N terminus (22)(23)(24). MLCK has been shown to be expressed in IHCs (21).…”