Although 67LR isolated from cells migrates at approx. 67 kDa
on denaturing polyacrylamide gels, the corresponding gene only
encodes a protein of approx. 33 kDa [5], which migrates with
a molecular mass of approx. 37 kDa on SDS/PAGE. This gene
product, 37LRP, is recognized by some (but not all) antibodies
raised against 67LR [25]. The cause of this anomaly has not
been determined. The most common explanations for such behaviour
in proteins are post-translational covalent modification
or the formation of very tightly associated homo- or heterodimers.
Although reducing and denaturing gel systems, such
as SDS/PAGE, normally completely disassociate protein complexes
into their components, it is not unknown for very tightly
bound complexes to resist this, at least partially (for example,
see [26]). Interestingly, there is evidence for both these possibilities,
but the exact cause of the molecular mass discrepancy
remains a mystery. One hypothesis states that the 33 kDa protein
encoded by the cDNA clone is a precursor for 67LR which
undergoes some form of modification in order to become the
mature protein.