ANALYSIS OF EXPRESSION OF AN ALTERNATIVE LA (SS-B) CDNA AND LOCALIZATION OF THE ENCODED N-TERMINAL AND C-TERMINAL PEPTIDES

A deletion of an (A)-residue was detected in a cDNA encoding for the n uclear autoantigen La/SS-B. The cDNA was recently isolated from a cDNA library made from peripheral blood lymphocytes of a patient with prim ary Sjogren's Syndrome. The region, where the deletion occurred, repre sents a hot spot region in the La gene(s). It leads to a frame shift m utation and a premature stop codon eleven amino acids downstream of th e deletion site within one of the protease sensitive regions of the La protein. In spite of the frame shift mutation expression of full leng th La protein occurred efficiently in E. coli. Full length La protein was also made in SF9 cells infected with recombinant baculoviruses, al though the efficiency of full length protein production was less. Two major peptides with molecular weights of 29 kDa and 25 kDa were made. The size of these peptides was similar to the known proteolytic degrad ation products of La protein. The N-terminal 29 kDa fragment containin g the RNP consensus sequence located in the cytoplasm. The 25 kDa C-te rminal fragment containing the nuclear location signal entered in the nucleus and associated with nuclear speckles. In conclusion, the abili ty to (i) enter, (ii) remain in the nucleus and (iii) assemble with nu clear speckles resides in the C-terminal domain of La protein and does not depend on the N-terminal RNP-consensus motif.