Further Characterization Of Platelet Membrane Glycoproteins

The binding of ¹²⁵I-labelled lectins to major and minor platelet glycoproteins (GP) and their subunits has been investigated. Human platelets were isolated, washed, solubilized in sodium dodecyl sulphate (SDS) under non-reducing conditions and separated on 5, 7.5 and 10 % non-reduced/reduced 2-D polyacrylamide gels. The gels were incubated with ¹²⁵I-labelled lectins; Lens culinaris lectin (LCL), concanavalin A (ConA) wheat germ agglutinin (WGA) or Ricinus communis agglutinin (RCA-120), then washed extensively dried and exposed to X-ray film by indirect autoradiography. Surface-labelled platelets were similarly separated. WGA and RCA bound predominantly to GPIbα but also to two minor bands above and below it which were affected by neuraminidase treatment. One of them bound two ¹²⁵I-lectins (LCL and ConA) while GPIbα did not. Additional GP bands were detected by lectin binding and by surface-labelling beneath GPIIIβ (IV). With platelets labelled by the neuraminidase/galactose oxidase/NaB³H₄ method a GP was detected between Ila and Ilia which was not found with periodate/ NaB³H₄ labelling (not affected by reduction). Two spots on the diagonal bound LCL and ConA. GP Ibβ bound LCL more strongly than IIbp. GPIbp also bound WGA and RCA. GPIcβ apparently bound only ConA. GPIbβ and IIbβ were labelled equally strongly by surface labelling techniques, Icβ was apparently not labelled. Further GP subunits were detected one below Ibβ and IIbβ and another which originated in the GPVII region. These techniques demonstrate that the platelet surface is even more complex than previously thought.