Plant hybrid proline-rich proteins (HyPRPs) are putative cell wall proteins consisting, usually, of a repetitive proline-rich (PR) N-terminal domain and a conserved eight-cysteine motif (8 CM) C-terminal domain. Understanding the evolutionary dynamics of HyPRPs might provide not only insight into their so far elusive function, but also a model for other large protein families in plants.
We have performed a phylogenetic analysis of HyPRPs from seven plant species, including representatives of gymnosperms and both monocot and dicot angiosperms. Every species studied possesses a large family of 14–52 HyPRPs. Angiosperm HyPRPs exhibit signs of recent major diversification involving, at least in Arabidopsis and rice, several independent tandem gene multiplications. A distinct subfamily of relatively well-conserved C-type HyPRPs, often with long hydrophobic PR domains, has been identified. In most of gymnosperm (pine) HyPRPs, diversity appears within the C-type group while angiosperms have only a few of well-conserved C-type representatives. Atypical (glycine-rich or extremely short) N-terminal domains apparently evolved independently in multiple lineages of the HyPRP family, possibly via inversion or loss of sequences encoding proline-rich domains. Expression profiles of potato and Arabidopsis HyPRP genes exhibit instances of both overlapping and complementary organ distribution. The diversified non-C-type HyPRP genes from recently amplified chromosomal clusters in Arabidopsis often share their specialized expression profiles. C-type genes have broader expression patterns in both species (potato and Arabidopsis), although orthologous genes exhibit some differences.
HyPRPs represent a dynamically evolving protein family apparently unique to seed plants. We suggest that ancestral HyPRPs with long proline-rich domains produced the current diversity through ongoing gene duplications accompanied by shortening, modification or loss of the proline-rich domains. Most of the diversity in gymnosperms and angiosperms originates from different branches of the HyPRP family. Rapid sequence diversification is consistent with only limited requirements for structure conservation and, together with high variability of gene expression patterns, limits the interpretation of any functional study focused on a single HyPRP gene or a couple of HYPRP genes in single plant species.