A NOVEL MAMMALIAN MYOSIN-I FROM RAT WITH AN SH3 DOMAIN LOCALIZES TO CON-A-INDUCIBLE, F-ACTIN-RICH STRUCTURES AT CELL-CELL CONTACTS

In an effort to determine diversity and function of mammalian myosin I molecules, we report here the cloning and characterization of myr 3 ( third unconventional myosin from rat), a novel mammalian myosin I from rat tissues that is related to myosin I molecules from protozoa. Like the protozoan myosin I molecules, myr 3 consists of a myosin head dom ain, a single light chain binding motif, and a tail region that includ es a COOH-terminal SH3 domain. However, myr 3 lacks the regulatory pho sphorylation site present in the head domain of protozoan myosin I mol ecules. Evidence was obtained that the COOH terminus of the tail domai n is involved in regulating F-actin binding activity of the NH2-termin al head domain. The light chain of myr 3 was identified as the Ca2+-bi nding protein calmodulin. Northern blot and immunoblot analyses reveal ed that myr 3 is expressed in many tissues and cell lines. Immunofluor escence studies with anti-myr 3 antibodies in NRK cells demonstrated t hat myr 3 is localized in the cytoplasm and in elongated structures at regions of cell-cell contact. These elongated structures contained F- actin and alpha-actinin but were devoid of vinculin. Incubation of NRK cells with Con A stimulated the formation of myr 3-containing structu res along cell-cell contacts. These results suggest for myr 3 a functi on mediated by cell-cell contact.