NHP6A AND NHP6B, WHICH ENCODE HMG1-LIKE PROTEINS, ARE CANDIDATES FOR DOWNSTREAM COMPONENTS OF THE YEAST SLT2 MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAY

The yeast SLK1 (BCK1) gene encodes a mitogen-activated protein kinase (MAPK) activator protein which functions upstream in a protein kinase cascade that converges on the MAPK Slt2p (Mpk1p). Dominant alleles of SLK1 have been shown to bypass the conditional lethality of a protein kinase C mutation, pkc1-DELTA, suggesting that Pkc1p may regulate Slk1 p function. Slk1p has an important role in morphogenesis and growth co ntrol, and deletions of the SLK1 gene are lethal in a spa2-DELTA mutan t background. To search for genes that interact with the SLK1-SLT2 pat hway, a synthetic lethal suppression screen was carried out. Genes whi ch in multiple copies suppress the synthetic lethality of slk1-1 spa2- A were identified, and one, the NHP6A gene, has been extensively chara cterized. The NHP6A gene and the closely related NHP6B gene were shown previously to encode HMG1-like chromatin-associated proteins. We demo nstrate here that these genes are functionally redundant and that mult iple copies of either NHP6A or NHP6B suppress slk1-DELTA and slt2-DELT A. Strains from which both NHP6 genes were deleted (nhp6-DELTA mutants ) share many phenotypes with pkc1-DELTA, slk1-DELTA, and slt2-DELTA mu tants. nhp6-DELTA cells display a temperature-sensitive growth defect that is rescued by the addition of 1 M sorbitol to the medium, and the y are sensitive to starvation. nhp6-DELTA strains also exhibit a varie ty of morphological and cytoskeletal defects. At the restrictive tempe rature for growth, nhp6-DELTA mutant cells contain elongated buds and enlarged necks. Many cells have patches of chitin staining on their ce ll surfaces, and chitin deposition is enhanced at the necks of budded cells. nhp6-DELTA cells display a defect in actin polarity and often a ccumulate large actin chunks. Genetic and phenotypic analysis indicate s that NHP6A and NHP6B function downstream of SLT2. Our results indica te that the Slt2p MAPK pathway in Saccharomyces cerevisiae may mediate its function in cell growth and morphogenesis, at least in part, thro ugh high-mobility group proteins.