Merkulov G V, Swiderek K M, Brachmann C B, Boeke J D
Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21025, USA.
J Virol. 1996 Aug;70(8):5548-56. doi: 10.1128/JVI.70.8.5548-5556.1996.
Cleavage of the Gag and Gag-Pol polyprotein precursors is a critical step in proliferation of retroviruses and retroelements. The Ty1 retroelement of Saccharomyces cerevisiae forms virus-like particles (VLPs) made of the Gag protein. Ty1 Gag is not obviously homologous to the Gag proteins of retroviruses. The apparent molecular mass of Gag is reduced from 58 to 54 kDa during particle maturation. Antibodies raised against the C-terminal peptide of Gag react with the 58-kDa polypeptide but not with the 54-kDa one, indicating that Gag is proteolytically processed at the C terminus. A protease cleavage site between positions 401 and 402 of the Gag precursor was defined by carboxy-terminal sequencing of the processed form of Gag. Certain deletion and substitution mutations in the C terminus of the Gag precursor result in particles that are two-thirds the diameter of the wild-type VLPs. While the Ty1 protease is active in these mutants, their transposition rates are decreased 20-fold compared with that of wild-type Ty1. Thus, the Gag C-terminal portion, released in the course of particle maturation, probably plays a significant role in VLP morphogenesis and Ty1 transposition.
Gag和Gag-Pol多蛋白前体的切割是逆转录病毒和逆转座子增殖过程中的关键步骤。酿酒酵母的Ty1逆转座子形成由Gag蛋白构成的病毒样颗粒（VLP）。Ty1 Gag与逆转录病毒的Gag蛋白没有明显的同源性。在颗粒成熟过程中，Gag的表观分子量从58 kDa降至54 kDa。针对Gag C末端肽产生的抗体与58 kDa多肽发生反应，但不与54 kDa多肽发生反应，这表明Gag在C末端进行了蛋白水解加工。通过对加工后的Gag形式进行羧基末端测序，确定了Gag前体第401位和402位之间的蛋白酶切割位点。Gag前体C末端的某些缺失和取代突变导致颗粒直径仅为野生型VLP的三分之二。虽然Ty1蛋白酶在这些突变体中具有活性，但其转座率与野生型Ty1相比降低了20倍。因此，在颗粒成熟过程中释放的Gag C末端部分可能在VLP形态发生和Ty1转座中发挥重要作用。
