Suppressors of mdm20 in yeast identify new alleles of ACT1 and TPM1 predicted to enhance actin-tropomyosin interactions

The actin cytoskeleton is required for many aspects of cell division in yea st, including mitochondrial partitioning into growing buds (mitochondrial i nheritance). Yeast cells lacking MDM20 function display defects in both mit ochondrial inheritance and actin organization, specifically, a lack of visi ble actin cables and enhanced sensitivity to Latrunculin X. mdm20 mutants a lso exhibit a temperature-sensitive growth phenotype, which we exploited to isolate second-site suppressor mutations. Nine dominant suppressors select ed in an mdm20/mdm20 background rescue temperature-sensitive growth defects and mitochondrial inheritance defects and partially restore actin cables i n haploid and diploid mdm20 strains. The suppressor mutations define new al leles of ACT1 and TPM1, which encode actin and the major form of tropomyosi n in yeast, respectively. The ACT1 mutations cluster in a region of the act in protein predicted to contact tropomyosin, suggesting that they stabilize actin cables by enhancing actin-tropomyosin interactions. The characterist ics of the mutant ACT1 and TPM1 alleles and their potential effects on prot ein structure and binding are discussed.