TROPOMYOSIN IS ESSENTIAL IN YEAST, YET THE TPM1 AND TPM2 PRODUCTS PERFORM DISTINCT FUNCTIONS

Sequence analysis of chromosome IX of Saccharomyces cerevisiae revealed an open reading frame of 166 residues, designated TPM2, having 64.5% sequence identity to TPM1, that encodes the major form of tropomyosin in yeast. Purification and characterization of Tpm2p revealed a protein with the characteristics of a bona fide tropomyosin; it is present in vivo at about one sixth the abundance of Tpm1p. Biochemical and sequence analysis indicates that Tpm2p spans four actin monomers along a filament, whereas Tpm1p spans five. Despite its shorter length, Tpm2p can compete with Tpm1p for binding to F-actin. Over-expression of Tpm2p in vivo alters the axial budding of haploids to a bipolar pattern, and this can be partially suppressed by co-overexpression of Tpm1p. This suggests distinct functions for the two tropomyosins, and indicates that the ratio between them is important for correct morphogenesis. Loss of Tpm2p has no detectable phenotype in otherwise wild type cells, but is lethal in combination with tpml Delta. Over-expression of Tpm2p does not suppress the growth or cell surface targeting defects associated with tpml Delta, so the two tropomyosins must perform an essential function, yet are not functionally interchangeable. S. cerevisiae therefore provides a simple system for the study of two tropomyosins having distinct yet overlapping functions.