Comparative analysis of PSK peptide growth factor precursor homologs

We have identified homologs of the sulfated peptide growth factor precursor gene OsPSK, which induces proliferation of low-density cell cultures similar to the feeder effect observed with culture medium from rapidly growing cells. PSK homologs are present in both monocots and dicots as small gene families. Overall, amino acid sequences of PSK homologs are little conserved with the exception of the signature motif Cx((4-9))[E/D/Q]xCx((2))RRx((3-4))AH[T/L/V]DYIYTQ. The OsPSK growth factor shares only the PSK domain HTDYIYTQ with the identified PSK-related genes. The YIYTQ pentapeptide in OsPSK is sulfated to yield the active mitogen. High conservation of PSK proteins was observed with respect to secondary structure and biochemical properties of defined domains. All PSK precursor proteins possess a hydrophobic leader sequence, which is in accordance with the finding that OsPSK is secreted. Secondary structure analysis of PSKs indicated that the post-leader sequence is structured as three alpha-helices with a turn between helices two and three, which may have implications for precursor processing. In PSK proteins, with the exception of OsPSK, an acidic region precedes and partially overlaps with the signature motif, and the YIYTQ pentapeptide itself is bracketed by two pairs of basic residues which may be important for precursor processing. The rice PSK gene family was shown to be differentially regulated with respect to tissue specificity, response to growth inducing submergence treatment and sensitivity to cycloheximide. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.