The 100-kDa chain of nicein/kalinin is a laminin B2 chain variant

We have isolated the basement membrane component nicein and performed rotary-shadow analyses using electron microscopy that showed the presence of two forms (I and II) of the protein. Molecular cloning of the cDNA that codes for the 100-kDa chain of the protein revealed that the sequence matches those independently identified for the 105-155-kDa subunit of kalinin, a recently identified basement membrane component. These data demonstrate that nicein and kalinin contain an identical chain. The length of the open reading frame in the cDNA (approximately 5200 nucleotides) and amino acid sequence obtained from the N-terminus of the 105-kDa kalinin chain showed the occurrence of a precursor polypeptide. This immature polypeptide is probably related to form I, observed by rotary shadowing, while the mature form is related to form II. It is noteworthy that nicein/kalinin subunits share discrete sequence similarities with the B2 chain of human laminin, but with a cleavage occurring within domain III that eliminates domains IV and V from the final product. The sequence of this subunit is nearly identical to that of B2t, a recently described polypeptide supposed to be related to a new laminin variant. Since nicein/kalinin expression is specifically impaired in the severe genodermatosis Herlitz junctional epidermolysis bullosa, the role and structure of this tissue-restricted laminin variant is crucial for the understanding of epidermal-dermal adhesion.