Analysis of the transmembrane domain of influenza virus neuraminidase, a type II transmembrane glycoprotein, for apical sorting and raft association

Influenza virus neuraminidase (NA), a type II transmembrane protein, is dir ectly transported to the epical plasma membrane in polarized MDCK cells. Pr eviously, it was shown that the transmembrane domain (TMD) of NA provides a determinant(s) for apical sorting and raft association (A. Kundu, R. T. Av alos, C. M. Sanderson, and D. P. Nayak, J. Virol. 70:6508-6515, 1996). In t his report, we have analyzed the sequences in the NA TMD involved in apical transport and raft association by making chimeric TMDs from NA and human t ransferring receptor (TR) TMDs and by mutating the NA TMD sequences. Our re sults show that the COOH-terminal half of the NA TMD (amino acids [aa] 19 t o 35) was significantly involved in raft association, as determined by Trit on X-100 (TX-100) resistance. However, in addition, the highly conserved re sidues at the extreme NH, terminus of the NA TMD were also critical for TX- 100 resistance. On the other hand, 19 residues (aa 9 to 27) at the NH, term inus of the NA TMD were sufficient for epical sorting. Amino acid residues 14 to 18 and 27 to 31 had the least effect on apical transport, whereas mut ations in the amino acid residues 11 to 13, 23 to 26, and 32 to 35 resulted in altered polarity for the mutant proteins. These results indicated that multiple regions in the NA TMD were involved in apical transport. Furthermo re, these results support the idea that the signals for apical sorting and raft association, although residing in the NA TMD, are not identical and va ry independently and that the NA TMD also possesses an apical determinant(s ) which can interact with apical sorting machineries outside the lipid raft .