Structure-function analysis of Knr4/Smi1, a newly member of intrinsically disordered proteins family, indispensable in the absence of a functional PKC1-SLT2 pathway in Saccharomyces cerevisiae.

The coordination between cell wall synthesis and cell growth in the yeast Saccharomyces cerevisiae implicates the PKC1-dependent MAP kinase pathway. KNR4, encoding a 505 amino acid long protein, participates in this coordination, since it displays synthetic lethality with all the members of the PKC1 pathway and shows physical interaction with Slt2/Mpk1. The recent finding that KNR4 interacts genetically or physically with more than 100 partners implicated in different cellular processes raised the question of how these interactions may occur and their physiological significance. This called for an in-depth structure-function analysis of the Knr4 protein, which is reported in the present paper. Computational analysis supported by biochemical and biophysical data characterize Knr4 as a newly identified member of the growing family of intrinsically disordered proteins. Despite disordered regions that are located at the N- and C-termini and are probably responsible for fine regulatory function; this protein contains a structured central core (amino acid residues 80-340) that is able to restore wild-type phenotypes of knr4Delta mutant in stress conditions. However, this fragment was unable to complement synthetic lethality between knr4 mutations and deletions of genes encoding protein kinases of the PKC1-dependent pathway. For these crucial events to occur, the presence of the N-terminal part of Knr4 protein is indispensable. Moreover, we demonstrate that this protein is essential for cell viability in the absence of a functional Pkc1-Slt2 pathway, since the lethality caused by KNR4 deletion in such a genetic background could not be compensated by overexpression of any gene from yeast genomic libraries. Copyright 2008 John Wiley & Sons, Ltd.