A novel family of plant splicing factors with a Zn knuckle motif: examination of RNA binding and splicing activities

An important group of splicing factors involved in constitutive and alterna tive splicing contain an arginine/serine (RS)-rich domain. We have previous ly demonstrated the existence of such factors in plants and report now on a new family of splicing factors (termed the RSZ family) from Arabidopsis th aliana which additionally harbor a Zn knuckle motif similar to the human sp licing factor 9G8. Although only around 20 kDa in size, members of this fam ily possess a multi-domain structure. In addition to the N-terminal RNA rec ognition motif (RRM), a Zn finger motif of the CCHC-type is inserted in an RGG-rich region; all three motifs are known to contribute to RNA binding. T he C-terminal domain has a characteristic repeated structure which is very arginine-rich and centered around an SP dipeptide. One member of this famil y, atRSZp22, has been shown to be a phosphoprotein with properties similar to SR proteins. Furthermore, atRSZp22 was able to complement efficiently sp licing deficient mammalian S100 as well as h9G8-depleted extracts. RNA bind ing assays to selected RNA sequences indicate an RNA binding specificity si milar to the human splicing factors 9G8 and SRp20. Taken together, these re sult show that atRSZp22 is a true plant splicing factor which combines stru ctural and functional features of both h9G8 and hSRp20.