No.42 - è¾²æ¥­ç”Ÿç‰©è³‡æºç ”ç©¶æ‰€

PEPTIDE PLANT HORMONE, PHYTOSULFOKINE
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of intact plants, indicating that PSK expression is not limited to regions in which cells actively
divide and differentiate. (Yang H. et al. 2001) To study the function of PSK in plants, a large
number of transformation experiments have been performed using Arabidopsis. However, loss-offunction
techniques have not produced visible, directly informative phenotypes, suggesting
functional redundancy between these 5 PSK genes in Arabidopsis. A definite picture will only
emerge when combinations of all five knockouts are available. Overexpression of PSK slightly
promotes callus formation in vitro in the presence of auxin/cytokinin, (Yang H. et al. 2001) but
does not affect the growth of seedlings.
Structure-activity relationships of PSK
Derivatization of peptide hormones with biochemical tags such as photoactivatable groups has
been used in characterization and purification of hormone receptors. (Hazum E. 1983) A key factor
in the use of such functional groups is the ability to derivatize peptides without loss of binding
activity or biological activity.
To identify the active core of PSK, we synthesized several PSK analogs by solid phase peptide
synthesis and direct sulfation of the peptide-resin using dimethylformamide-sulfurtrioxide
complex. (Matsubayashi Y. et al. 1996) As shown in Fig. 5, N-terminal tetrapeptide and tripeptide
of PSK retained 8% and 20% of the activity of the parent pentapeptide, respectively, but N-
terminal dipeptide showed no activity. Deletion of the sulfate groups of Tyr 1 and Tyr 3 resulted in
compounds with 0.6% and 4% of the activity of PSK, respectively, indicating that the sulfate group
of Tyr 1 is more important than that of Tyr 3 for activity. In contrast, the N-terminal-truncated analog
and an unsulfated analog exhibited no activity. Thus, the N-terminal tripeptide fragment Tyr(SO3H)-
Ile-Tyr(SO3H) has been identified as the active core of PSK.
A popular method for covalently linking functional groups to a peptide involves the use of
activated esters of the functional groups, which react with primary amines to form amide bonds.
However, modification of the N-terminal amino group of PSK by addition of Gly strongly
decreases its biological activity. (Matsubayashi Y. et al. 1996) Thus, functional derivatization of
PSK requires incorporation of an additional primary amino group at the C-terminal region, which
is less involved in PSK activity than the N-terminal. To fulfill these requirements, several Alasubstituted
PSK analogs were tested for activity, and the analog [Ala 5 ]PSK and [Lys 5 ]PSK was
found to possess binding activity equal to that of PSK (Fig. 5). (Matsubayashi Y. et al. 1999)
Interestingly, [Lys 5 ]PSK retained significant activity after derivatization of the side chain of Lys 5 by
biotin, even when a very long spacer chain was inserted between the amino group of Lys 5 and
the carboxyl group of biotin. This finding provided the breakthrough in a series of experiments
conducted with the aim of visualization and purification of PSK receptors.