ANALYSIS OF THE CARBOXYL-TERMINAL PEROXISOMAL TARGETING SIGNAL-1 IN AHOMOLOGOUS CONTEXT IN SACCHAROMYCES-CEREVISIAE

Most peroxisomal matrix proteins contain a carboxyl-terminal tripeptid e that directs them to peroxisomes. Within limits, these amino acids m ay be varied, without loss of function, The specificity of this peroxi somal targeting signal (PTS1) is remarkable considering its small size and its relaxed consensus sequence. Moreover, several peroxisomal pro teins have a PTS1-like signal that does not fit the reported consensus sequence. Because many of these PTS1 variants seem to be functional i n a species-dependent or protein context-dependent manner, me investig ated the PTS1 requirements in a homologous context, using Saccharomyce s cerevisiae and endogenous peroxisomal malate dehydrogenase (MDH3). P eroxisomal import of the MDH3-PTS1 variants was tested qualitatively b y the ability to complement the Delta mdh3 mutant and quantitatively b y subcellular fractionation. We observed efficient import of MDH3 into peroxisomes with a large variety of PTS1 tripeptides. Many of these v ariants do not fit the observed PTS1 requirements for heterologously e xpressed proteins, which suggests that additional domains in the prote in may be of decisive importance whether or not a certain PTS1 variant is recognized by the components of the peroxisomal import machinery. Because we show that dimerization of MDH3 precedes import into the org anelle, these domains are most likely conformational domains.