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Abstract:
The intermediate filament (IF) subunit protein vimentin is efficiently cleaved in vitro by purified human immunodeficiency virus type 1 (HIV-1) protease. Immunological data confirm that identical sites are cleaved when vimentin is polymerized into filaments or occurs as protofilaments. Preformed filaments require 10 times more protease to achieve the same extent of cleavage seen with protofilaments, suggesting that the cleavage sites are partially masked in IFs. The primary cleavage gives rise to molecule lacking most of the tail domain and which not only remains in preformed filaments, but also is capable of polymerizing into essentially normal 10 nm filaments. However, these filaments of the vimentin primary cleavage product show a propensity to form large lateral aggregates. The three secondary cleavage products of vimentin additionally lack portions of the head domain, are almost quantitatively released from preformed filaments and are not capable of forming filaments de novo. These results confirm and extend previous data obtained with desmin and provide a limit for that portion of the tail domain of type III IF subunit proteins that play a role in IF formation and stability. Microinjection of HIV-1 protease into cultured human skin fibroblasts resulted in a large increase in the percentage of cells with an altered and abnormal distribution of vimentin IFs. Most commonly, the IFs were observed to have collapsed into a clump with a juxtanuclear localization. The efficient cleavage of vimentin observed in vitro and the ability of microinjected HIV-1 protease to alter IF distribution in vivo suggest that IF proteins may serve as substrates within HIV-1 infected cells and may play a role in viral infection.
