Studies of two antigenic forms of Ld with disparate beta 2-microglobulin (beta 2m) associations suggest that beta 2m facilitate the folding of the alpha 1 and alpha 2 domains during de novo synthesis.

Sequential precipitation analysis of BALB/c Ag revealed two distinct antigenic forms of the Ld molecule distinguished by their reactivity with mAb 30-5-7. A similar analysis of Ag from the Ld transfectant T1.1.1 confirmed that both forms of Ld are products of the Ld gene. The 30-5-7+ form of Ld was found to be capable of association with beta-2 microglobulin (beta 2m) but could also exist as a free H chain, whereas the 30-5-7- form of Ld was incapable of beta 2m association. Unexpectedly, this latter form of Ld showed oligosaccharide maturation as well as cell surface expression, although less efficiently than the 30-5-7+ form of Ld. Pulse-chase experiments demonstrated that these two forms of Ld do not share a precursor-product relationship, but rather their distinguishing structures are fixed during de novo synthesis in the endoplasmic reticulum and remain constant throughout maturation and expression. Thus, beta 2m association is not an absolute requirement for intracellular transport and expression on the plasma membrane even in beta 2m+ cell types. Furthermore, in the context of other recent studies of Ld and Db, our results suggest that beta 2m plays a key role in folding the outer domains of class I molecules during de novo synthesis. It is speculated that beta 2m may provide a support structure analogous to a class II second domain, on which the class I binding site can be properly formed.