MPSBC investigators have determined the crystal structure of the CaaX protease Ste24p from Saccharomyces mikatae. This structure was published in the March 29 (2013) issue of Science (PMID: 23539602) along with a “perspectives” commentary (A protease for the ages). It was also the subject of an APS science highlight.
Ste24p is a zinc metalloprotease and an integral membrane protein. It is comprised of seven transmembrane helices that surround a large cavity (~14,000Å3) contained nearly completely within the membrane interior. The transmembrane helices range from 28 to 44 residues in length, and form a fenestrate surface with gaps of up to 10Å diameter between helices. The cavity is sufficient to contain a 10kDa protein or 450 water molecules. Helices III-VII are kinked or sharply curved, with helices III and IV containing proline residues at the sites of the kinks. Helix VI contains the zinc metalloprotease HEXXH motif.
Function of CaaX proteases. CaaX proteases attach isoprenoid groups to proteins via cysteine residues of CaaX acceptor sequences in which the cysteine attachment site is followed by two aliphatic amino acid residues and one unspecified residue at the protein C terminus. Isoprenylation is generally accompanied by two subsequent processing steps ̶ proteolytic cleavage of the aaX residues and carboxymethylation of the newly exposed carbonyl group of the modified cysteine residue. Some isoprenylated proteins also undergo additional proteolytic processing, including an additional cleavage by the same protease that initially removes the aaX residues. Ste24p is a type I prenyl protease that catalyzes two proteolytic steps in the maturation of yeast mating pheromone a-factor.
Medical relevance. A human ortholog of Ste24p, ZMPSTE24 (Zinc Metalloprotease STE24), can complement the full function of yeast Ste24p. The only known substrate for ZMPSTE24 is prelamin A, the precursor to the nuclear intermediate filament protein lamin A. Lamins provide mechanical stability to the nuclear envelope, function as scaffolds for localization of other proteins and for cytoskeletal attachment, regulate chromatin, and are implicated in transcription and DNA repair and replication. Mutations in either ZMPSTE24 or the processing site of prelamin A are associated with a spectrum of premature-aging diseases referred to as progeria. The severity of different forms of progeria is reported to be correlated with the extent of loss of ZMPSTE24 activity. In addition, ZMPSTE24 and yeast Ste24p are inhibited by antiviral drugs designed to target the HIV aspartyl protease, and this off-target interaction may give rise to some of the severe side effects of these drugs.
The structure of ZMPSTE24 was determined concurrently by the Structural Genomics Consortium ( PMID: 23539603
) and published back-to-back with MPSBC’s Ste24p structure in Science
. We anticipate that information from these structures will be utilized in several ways. First, their novel folds will provide invaluable input for the development and testing of mechanistic hypotheses. (For example, we have already postulated a processive mechanism of substrate insertion, translocation and ejection.) Secondly, since some of the very severe side-effects of HIV protease inhibitors have been implicated to arise from their off-target inhibition of this CaaX protease, (future) structures of complexes of HIV protease inhibitors with Ste24 could possibly be utilized for “negative design” of HIV protease inhibitors with reduced off-target side effects.
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