The complete deduced amino acid sequence and mRNA expression of human unconventional myosin-IC (HuncM-IC) are described. Sequencing of overlapping cDNA clones reveals a message of 4666 nucleotides with a single open reading frame predicted to encode a 127 kDa protein of 1109 amino acids. HuncM-IC is composed of three discrete regions: a characteristic N-terminal myosin head with predicted actin and ATP-binding sites; a neck domain with an "IQ motif", predicted to bind a single light chain; and a C-terminal tail with a putative membrane-binding site. In addition, the tail contains an src-homology 3 domain. The presence of a single IQ motif and an src-homology 3 domain is reminiscent of "long-tailed" myosins-I from amoeboid organisms, a supposition confirmed by multiple sequence alignment. Northern blot analysis of human tissues shows that HuncM-IC is ubiquitously expressed, with the highest levels in kidney, prostate, colon, liver and ovary. The results show that "amoeboid" myosins-I are not restricted to amoeboid organisms, rather they are expressed in the metazoa as well.