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A putative functional domain of human placental alkaline phosphatase predicted from sequence comparisons.

There are some reasons to speculate the existence of internally repeated sequences in ras proto-oncogene and oncogene products (p21s). p21s have not only cysteine proteinase inhibitor activity [1,2] but some amino acid sequence homology with cystatins, which are also cysteine proteinase inhibitors [1]. However, p21 is composed of 189 amino acid residues, more than cystatins; most cystatins are composed of less than 120 amino acids [3-12]. There are other types of cysteine proteinase inhibitors, e.g. kininogen [13-16] and ac2-HS-glycoprotein [1 7,18]. 'Low-molecular-w-eight' kininogen is composed of approx. 430 amino acid residues and has three internally repeated sequences, two of which are active in inhibition of cysteine proteinases [14-16]. The A chain of a2-HS-glycoprotein is composed of 282 amino acid residues [17] and has two internally repeated sequences [18]. Furthermore, there are similar characteristics between p21 and kininogen. Both p21 and kininogen inhibit cathepsin L more effectively than they do cathepsins B and H, although their inhibitory activities against papain and calpain are completely different [1, 15,16,19]. Kininogen can stimulate growth of mouse embryonic cells when it is added to the culture medium [20]; p21 can also stimulate cell growth after it is microinjected into NIH 3T3 cells [21]. Thus it seems possible that internally repeated sequences may be found in p21 as well as in kininogen. The sequence Gln-Xaa-Val (Xaa represents any amino acid) seems highly conserved among the cystatin superfamily [3-9, 11-14]. There are two sequences of Gln-Xaa-Val in p21, which are 43Gln-Val-Val and '50Gln-Gly-Val. Direct alignment of these sequences made it very easy to explore internally repeated sequences in p21 (Fig. 1). The most similar are the regions