Expression, subcellular localization, and cloning of the 130-kDa regulatory subunit of myosin phosphatase in porcine aortic endothelial cells

In endothelial cells in situ and in primary culture, immunoblot analysis revealed an expression of the 180-kDa subunit of myosin phosphatase, similar to the myosin phosphatase targeting subunit (MYPT) of smooth muscle. Screening of an endothelial cell cDNA library yielded a clone encoding an NH₂-terminal fragment of 89.6 kDa, closely related to smooth muscle MYPT1. Two isoforms differing by a central insert of 56 residues were detected. In growing cells, MYPT1 was localized on stress fiber, but at confluence the localization pattern changed and MYPT1 was distributed close to the cell membrane and at cell-cell contacts. The membrane localization of MYPT1 suggested a target other than myosin and raised the possibility that MYPT1 may be involved in dephosphorylation of alternative substrate(s). These distinct mechanisms would also be dependent on the growth state of the endothelial cells, i.e., regulation of actin-myosin interactions in growing cells and an unknown function in cells at confluence.