Identification of a novel late domain in human parainfluenza virus type 3 matrix protein

Human parainfluenza virus type 3 (HPIV3) is a major cause of bronchiolitis and pneumonia in infants under 6 months of age. Like other enveloped RNA viruses, HPIV3 encodes a matrix (M) protein involved in the final assembly and budding steps of the viral life cycle. Illustrating the important role of M protein is its ability to induce its own budding from cells in the form of enveloped virus-like particles (VLPs). For related viruses, the feature of viral M proteins that allows this independent budding to occur is a critical amino acid sequence, called a late (L) domain, which interacts with the host cell vesicle-forming machinery. To identify the HPIV3 L domains, we selected four HPIV3 M protein sequences (PPKH, YLDV, KPEL, and YPNI) based on their sequence similarity to established L domains and made alanine-substitution mutants of each potential L domain sequence. When these mutant M proteins were expressed in cells, we found that disrupting the YLDV sequence caused a severe budding defect. To confirm these results, we then inserted these sequences into poorly budding L domain-deficient mutants of the Ebola virus matrix protein. Consistent with our previous findings, the YLDV-containing VP40-delta N13 mutant was able to restore budding efficiency to levels on par with wild-type VP40. These findings provide strong evidence to support the likelihood that the YLDV sequence of HPIV3 M protein functions as an L domain