The small GTPase Rho3 and the diaphanous/formin For3 function in polarized cell growth in fission yeast

Abstract

We identified a novel Rho gene rho3+ and studied its interaction with diaphanous/formin for3+ in the fission yeast Schizosaccharomyces pombe. Both rho3 null cells and for3 null cells showed defects in organization of not only actin cytoskeleton but also cytoplasmic microtubules (MTs). rho3 for3double null cells had defects that were more severe than each single null cell: polarized growth was deficient in the double null cells. Function of For3 needed the highly conserved FH1 and FH2 domains, an N-terminal region containing a Rho-binding domain, and the C-terminal region. For3 bound to active forms of both Rho3 and Cdc42 but not to that of Rho1. For3 was localized as dots to the ends of interphase cells and to the mid-region in dividing cells. This localization was probably dependent on its interaction with Rho proteins. Overexpression of For3 produced huge swollen cells containing depolarized F-actin patches and thick cytoplasmic MT bundles. In addition, overexpression of a constitutively active Rho3Q71L induced a strong defect in cytokinesis. In conclusion, we propose that the Rho3-For3 signaling system functions in the polarized cell growth of fission yeast by controlling both actin cytoskeleton and MTs.

From Function to Shape: A Novel Role of a Formin in Morphogenesis of the FungusAshbya gossypii

“…For3 is involved in cell polarity (Feierbach and Chang, 2001), Cdc12 is involved in cytokinesis (Chang et al, 1997) and Fus1 is necessary for cell fusion during mating (Petersen et al, 1998a). Although S. pombe for3⌬ had no visible actin cables (Feierbach and Chang, 2001) or the cables were shorter in size and reduced in number (Nakano et al, 2002), cells were still able to grow in a polarized manner unlike a deletion of Agbni1. Different functions were also described for the baker's yeast formins ScBni1p and ScBnr1p (Pruyne et al, 2004).…”

“…In Schizosaccharomyces pombe, three formins exist which are also involved in cell polarity and cytokinesis. The protein SpCdc12p is involved in cytokinesis (Chang et al, 1997;Kovar et al, 2003), SpFus1p in cell fusion (Petersen et al, 1998b) and SpFor3p in cell polarity control via regulation of the actin and microtubule network (Feierbach and Chang, 2001;Nakano et al, 2002). The only formin family member described so far in a filamentous fungus is the SepA protein of Aspergillus nidulans.…”

Regulation of the Formin for3p by cdc42p and bud6p

“…Based upon sequence comparisons, for3p was categorized as a DAD-less formin (Higgs and Peterson, 2004). It has been shown to bind to activated rho3p and cdc42p but not other rhos, in two-hybrid assays (Nakano et al, 2002). However, the physiological significance of these interactions is not clear, especially because the function of these rhos in actin cable assembly has not yet been elucidated.…”

“…Fission yeast cells are rod-shaped and grow exclusively at cell tips. The formin for3p localizes to cell tips, and it is responsible for the assembly of a polarized array of actin cables (Feierbach and Chang, 2001;Nakano et al, 2002). These cables are bundles of short linear actin filaments largely oriented with their barbed ends facing the cell tip (Kamasaki et al, 2005).…”

“…We and others previously showed that the N terminus of for3p (amino acids [aa] 1-702; for3N) is sufficient to localize to cell tips (Nakano et al, Figure 2B). This N-terminal region also contains a Rho-binding site (Nakano et al, 2002). Residues in this region can be aligned, albeit poorly, to the DID and dimerization domains of mDia (Supplemental Figure 1).…”