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Abstract:
Myosin I motor molecules have been well characterized in the soil amoeba Acanthamoeba castellanii and the cellular slime mold Dictyostelium discoideum. However, no direct metazoa homologs have been identified so far. We have now cloned and sequenced a myosin I from rat tissue, called myr 3, which belongs to these "amoeboid type" myosin I molecules. Myr 3, a protein of 1107 amino acids, consists of a typical 70 kD motor domain, a regulatory domain with a single light chpi4 binding site ("IQ-motif) and a 42 kD tail domain with a SH3- domain at the C-terminus. Sequence comparison revealed that myr 3 is most similar to the myosins IB of Acanthamineba and Dictyostelium (55% identity). In contrast to these myosins it lacks the consensus phosphorylation sequence in the head domain. Phosphorylation of this site in the head domain of the "amoeboid" myosin I molecules activates their motor activity. Northern blot analysis showed the highest expression of the S.l kb mRNA in adult spleen and lung. Several fusion proteins containing different domains of myr 3 and two synthetic peptides were used to raise specific antibodies in rabbits. In immuno blotting experiments these antibodies recognized a protein of the expected molecular size in various tissues and cell lines. These data demonstrate that mammalian cells also contain at least one "amoeboid type" myosin I.
