Cell surface human α‐L‐fucosidase

Abstract

The acid a-l-fucosidase is usually found as a soluble component of lysosomes where fucoglycoconjugates are degraded. In the present investigation, we have demonstrated the existence of a cell surface protein with enzymatic a-l-fucosidase activity that crossreacts specifically with a rabbit anti-(a-l-fucosidase) Ig. By different approaches, this a-l-fucosidase, which represents 10±20% of the total cellular fucosidase activity, was detected in all the tested human cells (hemopoietic, epithelial, mesenchymal). Two bands of < 43±49 kDa were observed, although theoretical data support the possibility of having the same genetic origin that the known 50 to 55-kDa M r a-l-fucosidase. We speculate about an alternative traffic pathway for the plasma membrane a-l-fucosidase to work on the rapid turnover of glycoproteins.Keywords: a-l-fucosidase; plasma membrane; protein traffic; CD26; glycoprotein turnover.The lysosomal a-l-fucosidase (EC 3.2.1.51) is involved in the hydrolytic degradation of fucose-containing molecules. Mammalian a-l-fucosidases are multimeric forms of glycoproteins of < 53 kDa exhibiting optimal activity between pH values of 4 and 6.5 [1]. There appears to be a considerable degree of structural heterogeneity, both tissue-specific and within the same tissue. This is probably due to a different content in sialic acid residues [2,3], in addition to the two different alleles and genetic polymorphism encoded by the FUCA1 gene [4,5]. The significance of the FUCA2 gene is currently unknown [4±6].The absence or gross deficiency of a-l-fucosidase activity causes accumulation of fucoglycoconjugates, which leads to the genetic neurovisceral storage disease fucosidosis in mammals [7,8]. A new syndrome, previously known as leukocyte adhesion deficiency II, has been recently characterized as a generalized metabolic disease consisting of severe hypofucosylation of glycoconjugates [9]. Moreover, an abnormal intracellular and extracellular distribution of a-l-fucosidase is found, for example, in cystic fibrosis (decreased levels in antiserum and higher levels in skin fibroblasts) or in colon carcinomas (decreased levels in antiserum and in tumor tissue) [10±13]. In general, tissue differentiation and development through cell±cell recognition are modulated by sequential changes of the sugar chains of cell surface glycoproteins. As the expression or deletion of a-fucosyl residues linked at various positions of sugar chains of glycoproteins is one of these alterations, the role of a-l-fucosidase in these processes is of considerable interest.The majority (90±100%) of the a-l-fucosidase activity in almost all mammalian tissues investigated is in the soluble fraction [1], the human brain being the exception [14]. A serum form of a-l-fucosidase, which has attracted interest as a diagnostic marker in many clinical studies [15], has turned out to be similar to the tissue forms [4±6,16,17]. Very recently, a-l-fucosidase from rat testis and epididymal spermatozoa was found by immunocytochemistry to be associated with the outer p...

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