TOR2 is required for organization of the actin cytoskeleton in yeast

Abstract

The Saccharomyces cerevisiae gene TOR2 encodes a putative phosphatidylinositol kinase that has two essential functions. One function is redundant with TOR1, a TOR2 homolog, and is required for signaling translation initiation and early G1 progression. The second essential function is unique to TOR2. Here we report that loss of the TOR2-unique function disrupts polarized distribution of the actin cytoskeleton. A screen for dosage suppressors of a dominant negative TOR2 allele identified TCP20/CCT6, encoding a subunit of the cytosolic chaperonin TCP-1 that is involved in the biogenesis of actin structures. Overexpression of TCP20 restores growth and polarized distribution of the actin cytoskeleton in a tor2 mutant. TCP20 overexpression does not restore growth in a tor1 tor2 double mutant. We suggest that the unique function of the phosphatidylinositol kinase homolog TOR2 is required for signaling organization of the actin cytoskeleton during the cell cycle. TOR2, via its two functions, may thus integrate temporal and spatial control of cell growth.

Figure 1

Overexpression of tor2^kin− confers a growth defect that is suppressed by overexpression of TCP20. Wild-type cells (JK9-3da) transformed either with pTOR2kin− or with pTOR2kin− and pTCP20 (pAS26) were streaked on SD-URA-LEU and SGal/Gly-URA-LEU medium. The tor2^kin− allele is under control of the GAL1 promoter and therefore induced on galactose and repressed on glucose.

Figure 2

Overexpression of TCP20 suppresses the growth defect of a tor2^ts mutation but not of a tor1 tor2^ts double mutation. Wild-type TOR2 (JK9-3da), tor2^ts (SH121), and tor1 tor2^ts cells (SH221) carrying an empty vector or pTCP20 (pAS27) were streaked on YPD and incubated at 30°C or 37°C. tor2^ts and tor1 tor2^ts cells carrying an empty vector are not viable at 37°C.

Figure 3

Overexpression of TCP20 suppresses the actin organization defect of tor2^ts cells. Logarithmic cultures of wild-type TOR2 (JK9-3da) (A and B) and tor2^ts (SH121) cells carrying an empty vector (C and D) or pTCP20 (pAS27) (E and F) were grown at 30°C, shifted to 37°C for 6 h, fixed, stained with rhodamine phalloidin, and observed by fluorescence (A, C, and E) and Nomarski (B, D, and F) microscopy. tor2^ts cells carrying an empty vector have a random actin distribution.

Figure 4

Model of TOR2 unique and redundant functions controlling cell growth.