酿酒酵母非发酵代谢调控
2007-06-17 14:48:04   来源：网络数据库   评论：0 点击：

n with Snf1 kinase complex (ASC) domain involved in various protein–protein interactions (Yang et al. 1994; Jiang and Carlson 1997; Fig. 3). While an ASC domain is necessary and sufficient for interaction with Snf4 even in the absence of Snf1, KIS domains contact the C-terminal regulatory domain of Snf1 independently of Snf4(Fig. 3). The RD of Snf1 could be separated into subdomains, being responsible for binding to Snf4 (c-subunit) and to the KIS domain of b-subunits, respectively. Importantly, the ASC domain was shown to be bifunctional, as it is also necessary and sufficient for interaction with target proteins of the Snf1 kinase (demonstrated for Gal83 and the transcriptional activator Sip4; Vincent and Carlson 1999). Thus, b-subunits may provide a molecular scaffold and are also required for the definition of kinase substrates. Understanding the genuine regulatory function of b-subunits was initially complicated by the finding that the phenotype of a sip1 sip2 gal83 triple mutant apparently did not differ from that of a wild-type strain (Yang et al. 1994). However, more

recent results based on mutant alleles completely devoid of SIP1, SIP2 and GAL83 coding sequences clearly revealed a phenotype of the triple mutant which was indistinguishable from that of a snf1 deletion mutation (no utilization of raffinose, galactose and glycerol/ethanol as sole carbon sources; Schmidt and McCartney 2000). While deletion of the gene of a single b-subunit did not cause growth defects, double mutations led to the manifestation of a subset of snf phenotypes (deletion of SIP2 and GAL83 caused a defect with glycerol but not with raffinose). Snf1 kinase complexes containing only one of the three b-subunits could be purified and were shown to be enzymatically active (Nath et al. 2002). Presumably as a result of sequence conservation within their ASC domains, b-subunits are able to fulfil partially overlapping functions but may also confer target specificity.

However, this conclusion derived from the characterization of mutants remains to be confirmed biochemically.

Various regulatory inputs may control the Snf1 protein kinase complex. In contrast to its mammalian counterparts, Snf1 is not affected by AMP (Wilson et al.1996). An upstream acting kinase receives a signal possibly triggered by a high AMP/ATP ratio, leading to phosphorylation of Snf1 at threonine residue 210 (McCartney and Schmidt 2001). Snf1 could be deactivated by protein phosphatase 2A and subsequently reactivated by mammalian AMPK kinase or by an endogenous yeast Snf1-reactivating factor (Wilson et al.1996). However, the identity of this kinase is currently unknown. This mechanism is certainly not the sole signal input affecting Snf1 function. Recent results showed that the divergent N-termini of b-subunits may have an additional function in intracellular localization of the Snf1 kinase complex. Gal83 (but not Sip1 or Sip2) allowed nuclear import of the complex when yeast cells had to grow with a nonfermentable carbon source. In contrast, the complex was excluded from the nucleus when glucose or alternative sugars such as sucrose or galactose were available (Vincent et al. 2001b)