Glycoproteins 66 and 69 kDa of pollen tube wall: properties and distribution in angiosperms

Translational activity of tobacco pollen tubes is characterized by preferential synthesis and accumulation of glycoproteins 66 and 69 kDa. Screening of 19 species out of 15 families showed that the presence of one or both these proteins is a general feature of angiosperm pollen tubes, and their synthesis in tubes grown in vitro was detected in 16 out of 17 species investigated. Studies of 66 and 69 kDa proteins in selected species revealed the following similarities to the tobacco glycoproteins: affinity to lectin ConA, values of pI, noncovalent binding to pollen tube walls, and apparent Mr 58 kDa of their nascent precursor polypeptides resulting from inhibition of glycosylation. Partial cleavage of tobacco proteins 66 and 69 kDa with proteases produced almost identical patterns of fragments, indicating a homology between the two proteins. The digestion products of 66 and 69 kDa proteins were also identical in apple, but differed significantly from those produced in tobacco. Tobacco glycoprotein 69 kDa is encoded by the Ntp303 gene, homologous to another pollen specific and abundantly expressed gene Bp10 of Brassica napus. Pollen tubes of this species are shown here to exhibit high amounts of glycoproteins 66 and 69 kDa in their walls, like pollen tubes of tobacco. Computer analysis of predicted products of the two genes revealed their extensive homology, especially in conserved regions where functional domains are expected. A high level of identity was observed in putative Mr and pH values and in predicted phosphorylation, N-glycosylation, myristylation, and protein kinase sites. The results suggest early evolutionary origin, wide occurrence and functional similarity of genes encoding glycoproteins 66 and 69 kDa, and a specific role of these glycoproteins in the secretory pathway associated with polarized growth of angiosperm pollen tubes.