PST and STX are polysialyltransferases that form polysialic acid in the neural cell adhesion molecule (N-CAM), although it is not known why these two polysialyltransferases exist. In the present study, we have first isolated cDNA encoding human STX, which includes 5-untranslated sequence. Northern blot analysis, using this cDNA and PST cDNA previously isolated by us, demonstrated that PST and STX are expressed in different fetal and adult tissues. STX is primarily expressed in embryonic tissues, but only modestly in adult heart, brain, and thymus. PST, on the other hand, is continuously expressed in adult heart, brain, thymus, spleen, small and large intestines, and peripheral blood leukocytes. In various parts of adult brain, the relative amount of PST and STX appears to be substantially different depending on the regions. The analysis by in situ hybridization of mouse adult brain, however, suggests that polysialic acid in the hippocampal formation is synthesized by both STX and PST. HeLa cells doubly transfected with the isolated STX cDNA and N-CAM cDNA supported neurite outgrowth much better than HeLa cells expressing N-CAM alone. However, polysialic acid synthesized by PST appears to be a better substratum than that synthesized by STX. Moreover, the genes for PST and STX were found to reside at chromosome 5, band p21 and chromosome 15, band q26, respectively. These results, taken together, strongly suggest that PST and STX are expressed distinctly in tissue-specific and cell-specific manners and that they apparently have distinct roles in development and organogenesis.Polysialic acid is a developmentally regulated glycan composed of a linear homopolymer of ␣-2,8-linked sialic acid residues. Polysialic acid is mainly attached to the neural cell adhesion molecule (N-CAM) 1 and is more abundant in embryonic brain than adult brain (1-3). Presence of this large negatively charged carbohydrate modulates the adhesive property of N-CAM, and removal of polysialic acid increases binding between N-CAMs (4, 5). During embryonic development, the polysialylated embryonic from of N-CAM is restricted to migrating cells (6, 7), and the removal of polysialic acid from the N-CAM during embryonic development affects motor-axon projection patterns (8). Polysialylated N-CAM was also shown to attenuate cell-cell interactions mediated by other cell adhesion molecules (9 -11).Recently, we and others have cloned cDNAs encoding human, hamster, and mouse polysialyltransferases (PST for human, PST-1 for hamster, and ST8Sia IV for mouse, respectively) (12-14). The amino acid sequences of PST and PST-1 are more than 97% identical. Both PST and PST-1 directed the expression of polysialic acid on the cell surface. The same studies also revealed that PST is highly homologous to STX (sialyltransferase X, or ST8Sia II) (12, 13), and STX and PST have 59% identity at the amino acid level. STX was originally cloned as a developmentally regulated sialyltransferase from the rat fetal brain (15). Although the substrate specificity of STX was not kno...

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