The GDI1 genes from Kluyveromyces lactis and Pichia pastoris: cloning and functional expression in Saccharomyces cerevisiae

Development of a hexahistidine-3× FLAG-tandem affinity purification method for endogenous protein complexes in Pichia pastoris

We developed a method for efficient chromosome tagging in Pichia pastoris, using a useful tandem affinity purification (TAP) tag. The TAP tag, designated and used here as the THF tag, contains a thrombin protease cleavage site for removal of the TAP tag and a hexahistidine sequence (6× His) followed by three copies of the FLAG sequence (3× FLAG) for affinity purification. Using this method, THF-tagged RNA polymerases I, II, and III were successfully purified from P. pastoris. The method also enabled us to purify the tagged RNA polymerase II on a large scale, for its crystallization and preliminary X-ray crystallographic analysis. The method described here will be widely useful for the rapid and large-scale preparation of crystallization grade eukaryotic multi-subunit protein complexes.

Characterization of the sec1-1 and sec1-11 mutations

Sec1 proteins are implicated in positive and negative regulation of SNARE complex formation. To better understand the function of Sec1 proteins we have identified the nature of the temperature-sensitive mutations in sec1-1 and sec1-11. The sec1-1 mutation changes a conserved glycine(443) to glutamic acid. The sec1-11 mutation changes a highly conserved arginine(432) to proline. Based on homology and the crystal structure of the mammalian nSec1p, the corresponding amino acids localize to the 3b domain of nSec1p. Compared to the wild-type Sec1p the mutant proteins are less abundant even at the permissive temperature. Thus, the R432P and G443E mutations may cause structural alterations that affect folding and make the mutant proteins more susceptible to degradation. The remaining part is sufficient for growth and protein secretion at 24 degrees C and thus is likely to be properly folded. At 37 degrees C the mutant proteins become non-functional. In pulse-chase-type experiments the newly synthesized Sec1-1 and Sec1-11 proteins decayed similarly with the wild-type protein. Thus, the non-functionality of the mutant proteins cannot be explained by denaturation-induced degradation only. It is possible that the newly synthesized mutant proteins fold slowly and are susceptible to degradation before they have managed to fold and associate with other proteins. The mutant proteins were unable to interact with the Sec1p-interacting proteins Mso1p and Sso2p in the two-hybrid assay, even at the permissive temperature. These results localize sec1-1 and sec1-11 mutations to a domain of Sec1p and suggest a mechanism by which sec1-1 and sec1-11 cells become temperature-sensitive.