A limulus intracellular coagulation inhibitor type 2: Purification, characterization, cDNA cloning, and tissue localization

We described in a foregoing report findings on serpin, a serine protease inhibitor, newly identified in horseshoe crab (Tachypleus tridentatus) hemocytes and we name it limulus intracellular coagulation inhibitor, LICI (Miura, Y., Kawabata, S., and Iwanaga, S. (1994) J. Biol. Chem. 269, 542-547). This serpin specifically inhibits limulus lipopolysaccharide-sensitive serine protease, factor C̄. In ongoing studies on limulus serpin, we have found another inhibitor, LICI type-2 (LICI-2), which inhibits not only factor C̄ (k₁ = 7.1 × 10⁴ M^-1 s^-1) but also limulus clotting enzyme (k₁ = 4.3 × 10⁵ M^-1 s^-1). LICI-2 inhibits mammalian serine proteases, including α-thrombin, salivary kallikrein, plasmin, and tissue plasminogen activator. The inactivation of plasmin is the most rapid (k₁ = 1.2 × 10⁶ M^-1 s^-1). The purified LICI-2 is a single chain glycoprotein with an apparent M_r. = 42,000. A cDNA for LICI-2 was isolated and the open reading frame coded for a mature protein of 386 amino acids, of which 160 residues were confirmed by peptide sequencing. Although LICI-2 shows significant sequence similarity to the previous limulus serpin, LICI-1 (42% identity), LICI-2 contains a unique putative reactive site, -Lys-Ser-, distinct from that of LICI-1 (-Arg-Ser-). Northern blotting revealed expression of LICI-2 mRNA only in hemocytes, and not in heart, brain, stomach, intestine, coxal gland, and skeletal muscle. The immunoblot of large and small granule components with antiserum against purified LICI-2 suggests that LICI-2 is stored specifically in large granules, as in the case of LICI-1, and is released in response to external stimuli. We propose that the LICIs be classified into a new subfamily of intracellular serpins, regulated secretory serpins.