Poliovirus infection results in a number of host cell changes, including specific alterations in cellular proteins. This study further characterizes the cleavage of a cytoskeletal protein, microtubule-associated protein 4 (MAP-4) and investigates the identity of the viral protease which mediates its cleavage. MAP-4 cleavage by poliovirus was previously identified using a monoclonal antibody (M. Joachims and D. Etchison, 1992, J. Virol. 66, 5997-5804). In this study, MAP-4 cleavage was found to occur in cells infected by only some picornaviruses, poliovirus and human rhinovirus 14. Infection by other types of viruses, vesicular stomatitis virus and adenovirus, or by other types of picornaviruses, encephalomyocarditis virus, did not result in MAP-4 cleavage. To determine the viral mediator of MAP-4 cleavage, the effects of purified poliovirus proteases on MAP-4 integrity were examined by immunoblot. When MAP-4 substrates were incubated with concentrations of poliovirus 2A that were more than sufficient to induce p220 cleavage, there was no effect on MAP-4. However, when MAP-4 substrates were incubated with purified 3C protease (3Cpro), cleavage products were detected that were identical in size to those generated in vivo in poliovirus-infected cells; the use of a mutant 3C protease did not result in MAP-4 cleavage. Cleavage of MAP-4 was also demonstrated with purified 3CDpro, and the in vitro cleavage kinetics were examined. Indirect immunofluorescence revealed that MAP-4 cleavage also correlated with a marked "collapse" of microtubules during late infection, indicating a possible relationship between 3Cpro-mediated MAP-4 cleavage and changes in the microtubule system of infected cells.