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Fig. 7. Heterodimeric proteins form on the surface of CHO cells expressing LAMP and either OBCAM or CEPU-1. (A) Expression of LAMP in OBCAM-CHO cells destabilized the surface binding of LAMP-Fc. The arrowed cell, identified by GFP fluorescence, showed essentially no staining for LAMP-Fc despite its rounded, phase-bright morphology. Notice cells indicated with arrowheads that were also phase bright and stained intensely with LAMP-Fc. Expression of LAMP on the cell surface results in sequestration of free OBCAM into LAMP:OBCAM dimers, leaving no detectable OBCAM monomers or homodimers. (B) Expression of LAMP in OBCAM-CHO cells produced a small increase in fluorescence intensity when stained with OBCAM-Fc, possibly owing to increased affinity for LAMP:OBCAM dimers or overexpression of LAMP. Similar results were observed for CEPU-1-Fc (data not shown). (C) Expression of LAMP in CEPU-1-CHO cells also resulted in loss of LAMP-Fc binding (arrow). This is explained by the formation of LAMP:CEPU-1 dimers in cis, which destabilizes the binding of LAMP-Fc. (D) OBCAM-Fc binding increased as a result of LAMP expression, again because of the greater affinity of OBCAM-Fc for the heterodimer and/or the overexpression of LAMP. Similar results were observed for CEPU-1-Fc (data not shown). Bar, 25 µm.
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