Motile activities such as chemotaxis and phagocytosis, which occur in Dictyostelium cells lacking myosin II, may be dependent upon myosin I. To begin to explore this possibility, we have engineered a disruption of the Dictyostelium myosin I heavy chain (DMIHC) gene described recently (Jung, G., C. L. Saxe III, A. R. Kimmel, and J. A. Hammer III. 1989. Proc. Natl. Acad. Sci. USA. 86:6186-6190). The double-crossover, gene disruption event that occurred resulted in replacement of the middle approximate one-third of the gene with the neomycin resistance marker. The resulting cells are devoid of both the 3.6-kb DMIHC gene transcript and the 124-kD DMIHC polypeptide. DMIHC- cells are capable of chemotactic streaming and aggregation, but these processes are delayed. Furthermore, the rate of phagocytosis by DMIHC- cells is reduced, as assessed by growth rate on lawns of heat-killed bacteria and on the initial rate of uptake of FITC-labeled bacteria. Therefore, this Dictyostelium myosin I isoform appears to play a role in supporting chemotaxis and phagocytosis, but it is clearly not required for these processes to occur. Using a portion of the DMIHC gene as a probe, we have cloned three additional Dictyostelium small myosin heavy chain genes. Comparison of these four genes with three genes described recently by Titus et al. (Titus, M. A., H. M. Warrick, and J. A. Spudich. 1989. Cell Reg. 1:55-63) indicates that there are at least five small myosin heavy chain genes in Dictyostelium. The probability that there is considerable overlap of function between these small myosin isoforms indicates that multiple gene disruptions within a single cell may be necessary to generate a more striking myosin I- phenotype.

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