Mutation-function analysis in vivo of the nuclear localization signals of L2 minor capsid proteins of high risk HPV16 and low risk HPV11

Abstract

During the papillomavirus replication cycle, the L2 minor capsid protein enters the nucleus in the initial phase after uncoating of the incoming virions and in the productive phase when L2 together with L1 major capsid protein mediate the encapsidation of the newly replicated viral genome. L2 proteins of both high risk HPV16 L2 and low risk HPV11 L2 have two nuclear localization signals (NLSs): one at the N-terminus (nNLS) and one at the C terminus (cNLS). The purpose of these experiments is to determine the minimal mutations necessary to inhibit the function of the NLSs. In this study, subcellular localization of enhanced green fluorescent protein (EGFP) fusions with full length L2 and L2 mutants lacking either the cNLS (EGFP-L2ΔC), nNLS (EGFP-L2ΔN), or both NLSs (EGFP-L2ΔNΔC) was analyzed in HeLa cell transfection assays. Full length HPV16 L2 and HPV11 L2 proteins localize to the nucleus. For both HPV16 and 11 L2, each NLS could independently mediate nuclear import in vivo. EGFP fusions were also made with mutated nNLS (EGFP-L2ΔCSbN) or mutated cNLS (EGFP-L2ΔNSbC). Transfected HeLa cells were examined by fluorescence microscopy and quantitative studies were done. In both HPV16 and 11 L2 proteins, mutation of basic residues in either NLS inhibited its nuclear import ability.