An extensin isolated from
sugar beet (Beta vulgaris) cell
suspension cultures fulfills all criteria for membership of the extensin family save one, notably, lack of the ;diagnostic' pentamer Ser-Hyp-Hyp-Hyp-Hyp. However, sequence analysis of the major tryptic
peptides shows that
sugar beet extensin shares a motif in common with tomato extensin P1 but differs by the position of an insertion sequence [X] or [Y] which, in
sugar beet, splits the tetrahydroxyproline block: Ser-Hyp-Hyp-[X]-Hyp-Hyp-Thr-Hyp-
Val-Tyr-Lys, where [X] is [Val-His-Glu/Lys-
Tyr-Pro], while in tomato the insertion sequence [Y] = [
Val-Lys-
Pro-Tyr-
His-Pro] and, when it occurs, immediately follows the tetrahydroxyproline block: Ser-Hyp-Hyp-Hyp-Hyp-[Y]-Thr-Hyp-
Val-Tyr-Lys. Based on these data we reinterpret three highly repetitive
cDNA sequences, including
nodulin N75 from soybean and
wound-induced P33 of carrot, as extensins with split tetra(hydroxy)
proline blocks.