Functional and antigenic domains of the matrix (M1) protein of influenza A virus

Abstract

The membrane- and ribonucleocapsid (RNP)-binding domains of the matrix (M1) protein of influenza A virus (WSN strain) were partially mapped and characterized by reactivity with monoclonal antibodies (MAb) as well as by proteolytic cleavages and amino acid sequencing of the resulting peptides. Of two peptides formed by formic acid hydrolysis, a 9-kilodalton fragment at the amino-terminal third of the M1 protein was recognized by MAb M2-1C6 (to epitope 1), and a 15-kilodalton fragment at the carboxy-terminal two-thirds was recognized by MAb 289/4 (to epitope 2). Partial cleavage by staphylococcal V8 protease gave rise to a 16-kilodalton peptide, mapping to amino acid 8, which was recognized by MAbs to all three epitopes but rather weakly by MAb 904/6 to epitope 3. These studies suggest that epitope 1 of the M1 protein resides between amino acids 8 and 89, whereas epitopes 2, and possibly 3 are located between amino acids 89 and 141 or somewhat more carboxy distal. The intact M1 protein and its N-terminal 9- and 10-kilodalton peptides generated by formic acid or V8 protease cleavage, respectively, reconstituted with dipalmitoylphosphatidylcholine vesicles, but these N-terminal peptides had little effect on in vitro transcription of the RNP core. In sharp contrast, both intact M1 protein and the C-terminal 15-kilodalton formic acid fragment were able to inhibit viral transcription markedly. Moreover, 289/4 (to epitope 2) reversed this inhibited transcription significantly. These studies suggest that the lipid-binding domain of the M1 protein is located within the amino-terminal third, whereas the site involved in the interaction of the M1 protein with RNP cores is located within the carboxy-terminal two-thirds.