Abstract
Phytosulfokine is a peptide hormone isolated from conditioned medium of suspension culture and promotes cellular proliferation and differentiation by means of LRR-RLK, PSKR1. We analyzed binding kinetics of [3H]PSK to PSKR1 using carrot suspension cells expressing relatively high levels of PSKR1. Scatchard analysis of the binding data performed at 4oC revealed that binding constant and the number of receptor are 4.3 nM and 30,000 sites per cell, respectively. [3H]PSK binding was saturated within 2 h at 4oC and was decreased to the background level within 3 h after the addition of excess non-labeled PSK as a competitor. In contrast, when cells were incubated at 25oC, part of bound [3H]PSK was not dissociated by the addition of competitor, suggesting that [3H]PSK was internalized by receptor-mediated endocytosis. Internalization of [3H]PSK was inhibited by protein kinase inhibitor K-252a and PI3-kinase inhibitor wortmannin, but not by cycloheximide nor actinomycin D.
