The Na ϩ /H ϩ exchanger (NHE) is a plasma membrane transport protein found in a wide range of biological systems. NHE is involved in various functions including pH homeostasis, volume regulation, cell proliferation and transcellular Na ϩ absorption. This study reports immunodetection results obtained with antibodies generated against the C-terminus of the NHE of trout red blood cells, βNHE. Immunoblotting of cell membrane preparation reveals that βNHE is a protein with an apparent molecular mass of 95 kDa. Moreover enzymatic glycosidase treatment demonstrates that the antiporter is an N-glycosylated but not O-glycosylated protein. The primary structure of βNHE contains three putative N-glycosylation consensus sites (N-X-S/T) at Asn49, Asn338 and Asn378. Expression of βNHE in PS120 fibroblasts, a cell line which lacks an endogenous Na ϩ /H ϩ exchange, allows to determine the precise sites of glycosylation. The construction of a site-directed mutated βNHE antiporter, lacking the first predicted motif, shows that βNHE possesses an unique glycosylation site located on the first extracellular loop of the exchanger (Asn49). Expression of this deglycosylated antiporter shows that deglycosylation of the protein modifies neither the pH i dependency of the antiporter nor its hormonal stimulation. Keywords : Na ϩ /H ϩ exchanger ; red blood cell; glycosylation; protein kinase A.The Na ϩ /H ϩ exchanger (NHE) is a plasma membrane transport protein found in a wide range of biological systems. NHE is involved in various functions including pH homeostasis, volume regulation, cell proliferation and transcellular Na ϩ absorption [1]. The first Na ϩ /H ϩ exchanger was cloned by Sardet et al. [2] and called NHE1. At present, five other isoforms have been cloned: the trout red cell antiporter βNHE [3] and four mammalian subtypes : NHE2, NHE3, NHE4 and NHE5 [4Ϫ7] differently expressed in kidney, colon, small intestine and stomach epithelia [4Ϫ8]. NHE 2Ϫ4 represent the apically expressed NHE isoforms, NHE1 and NHE5 being expressed on the basolateral membrane and in non-epithelial cells.βNHE, the Na ϩ /H ϩ antiporter of trout (Oncorhynchus mykiss) red blood cells (RBC) is activated by adrenergic agonists. Addition of catecholamines, forskolin, or cAMP analogues to the RBC suspending medium strongly activates the antiporter: the Na ϩ influx increases enormously (100-fold) reaching its maximal value within 2Ϫ3 min.A peculiar βNHE property is that, following activation of the antiporter and despite the continuous presence of the activator, the Na ϩ /H ϩ exchange declines rapidly as a result of a desensitisation of the transport system and becomes refractory to new hormonal stimulation for 5 h [9]. This desensitisation, however, can be fully blocked and reversed by the protein phosphatase inhibitor okadaic acid (OA) [10]. From βNHE cloning and its functional expression in fibroblasts, it appears that the cytoplasmic C-terminal domain of βNHE contains two typical consensus sites for phosphorylation by the cAMP-dependent protein kinase A (PKA...