Occludin 1B, a variant of the tight junction protein occludin

Occludin and claudin are the major integral membrane components of the mamm alian tight junction. Although more than 11 distinct claudins have been ide ntified, only 1 occludin transcript has been reported thus far. Therefore, we searched by reverse transcription-PCR for occludin-related sequences in Madin-Darby canine kidney (MDCK) mRNA and identified a transcript encoding an alternatively spliced form of occludin, designated occludin 1B. The occl udin 1B transcript contained a 193-base pair insertion encoding a longer fo rm of occludin with a unique N-terminal sequence of 56 amino acids. Analysi s of the MDCK occludin gene revealed an exon containing the 193-base pair s equence between the exons encoding the original N terminus and the distal s equence, suggesting that occludin and occludin 1B arise from alternative sp licing of one transcript. To assess the expression and distribution of occl udin 1B, an antibody was raised against its unique N-terminal domain. Immun olabeling of occludin 1B in MDCK cells revealed a distribution indistinguis hable from that of occludin. Furthermore, occludin 1B staining at cell-to-c ell contacts was also found in cultured T84 human colon carcinoma cells and in frozen sections of mouse intestine. Immunoblots of various mouse tissue s revealed broad coexpression of occludin 1B with occludin. The wide epithe lial distribution and the conservation across species suggests a potentiall y important role for occludin 1B in the structure and function of the tight junction.