Suspension-cultured cells of carrot contain three populations of coated vesicles, associated with the plasma membrane (84–91 nm diameter), Golgi dictyosomes and the partially coated reticulum (61–73 nm diameter). These were observed by thin sectioning, dry-cleaving and rapid-freeze deep-etching of cells. Dissociation of clathrin coats with Tris, released triskelions that were morphologically identical with those from mammalian tissue. The triskelion arm length of carrot clathrin was greater (61nm versus 44–50 nm), but packaging results in clathrin cages of pentagons and hexagons of similar size to those from mammalian cells.
SDS-PAGE of Tris-released triskelion preparations revealed a complex of three polypeptides of 190, 60 and 57(x103)Mr. The 190x103Mr protein is the plant clathrin heavy chain, slightly larger than the mammalian heavy chain. The 60 and 57(x103)Mr bands showed the same sensitivities to protease treatment as mammalian light chains. Triskelion preparations containing these three proteins reassembled into polyhedral cages.
These results are discussed in relation to the structural organization of coated vesicles and clathrin cages in other systems.