Myosin II in retinal pigmented epithelial cells: evidence for an association with membranous vesicles

The goal of this study was to further characterize and identify possible functions for a cytoplasmic myosin II protein which we have isolated from retinal pigmented epithelial (RPE) cells. The nucleotide and deduced amino acid sequences are highly identical to non-muscle myosin heavy chain II-A (NMMHC II-A). However, this RPE myosin displays characteristics that are atypical of other myosins, including an affinity for carbohydrate and a C-terminal sequence extension, suggesting it may have a specialized function. In this study, reverse transcriptase-PCR using isoform-specific primers demonstrated that the RPE myosin and conventional NMMHC II-A have overlapping but distinguishable tissue expression profiles. To gain clues to function, subcellular distribution was determined in motile RPE cells using indirect immunofluorescence. In addition to subtle differences in localization that appeared to further distinguish this molecule from NMMHC II-A, these studies revealed a colocalization with phagocytosed intracellular vesicles. In vitro experiments suggest that the association in situ was not simply coincidental, because isolated vesicles interacted with the protein in cosedimentation assays. Taken together, our observations suggest the RPE myosin exhibits characteristics different from conventional myosin II-A and may function in intracellular vesicle transport.