ANALYSIS OF IGM STRUCTURES INVOLVED IN J-CHAIN INCORPORATION

J chain is associated with pentameric IgM and polymeric IgA. In IgM, J chain is disulfide bonded to the C575 residue of the mu-chain, locate d in the mu tail piece (mu tp). Previous studies indicated that mu tp is not sufficient to mediate J chain incorporation into polymeric Ig. In this study, we analyzed which other C mu domains are involved in J chain incorporation. Three altered forms of mouse IgM were analyzed: I gM lacking the C mu 1 domain, IgM in which the C mu 2 and C mu 3 domai ns were replaced by the hinge region and the C gamma 2 domain of IgG2b , and IgM, in which the C mu 4 domain was replaced by C gamma 3. We fo und that neither C mu 1, C mu 2, nor C mu 3 was absolutely required fo r J chain incorporation. The importance of C mu 4 could not be fully a nalyzed because the C gamma 3 replacement mutant failed to form polyme rs. Also, we found that the glycosylation site at asparagine 563 of mu tp was important for J chain incorporation. Disruption of this site b y replacement of either asparagine 563 by tyrosine or serine 565 by ph enylalanine resulted in diminished J chain incorporation and increased production of hexameric IgM. These results demonstrate the importance of structural elements located close to mu tp in the incorporation of J chain into IgM.