A potential sulfate transporter functioning downstream of the RB tumor suppressor pathway in Chlamydomonas

Su-Chiung Fang1 , James G. Umen2, Wirulda Pootakham3, Cristina Lopez2, Arthur Grossman3, and Chin-Lin Chung1

1) Biotechnology Center in Southern Taiwan, Academia Sinica, Tainan, 74146, Taiwan 2) Plant Biology Laboratory, The Salk Institute, La Jolla, CA 92307, USA 3) Department of Plant Biology, Carnegie Institution for Science, Stanford, CA 94305, USA

The retinoblastoma protein (RB) pathway is important to regulate the cell cycle progression during cell division. In Chlamydomonas reinhardtii, the RB homolog, MAT3, functions as a negative regulator of the cell cycle and its null allele results in defects in cell-size checkpoint control and production of tiny cells. Our previous genetic screen has identified SMT15, a potential sulfate transporter, functioning downstream of the MAT3 and its mutation partially suppresses small size of the mat3 mutant (smt mutants). Interestingly, SMT15 acts as a positive regulator as it suppresses the mat3 mutant but negatively regulate the size checkpoint in the presence of wild type MAT3. SMT15 belongs to a large SulP family that includes the SLC26 family anion transporters in mammalian systems. Transporters from SulP family found in plants, algae, and fungi appear to transport sulfate and several plant sulfate transporters are induced under sulfur starvation condition. Similarly, accumulation of SMT15 mRNA was induced in response to sulfur deprivation. We monitored the expression of sulfur acclimation genes in smt15-1 mutant and observed part of the program is mis-regulated. These findings suggest that SMT15 may be important to bridge the intracellular sulfur homeostasis to the cell cycle program. We are investigating the metabolic profile of smt15-1 mutant in response to sulfur acclimation and the impact of SMT15 on sulfur response will be discussed.

e-mail address of presenting author: scfang@gate.sinica.edu.tw