Translation of some maize small heat shock proteins is initiated from internal in-frame AUGs

Chaperone function of two small heat shock proteins from maize

Small heat shock proteins (sHsps) are molecular chaperones that protect cells from the effect of heat and other stresses. Some sHsps are also expressed at specific stages of development. In plants different classes of sHsps are expressed in the various cellular compartments. While the Class I (cytosolic) sHsps in wheat and pea have been studied extensively, there are fewer experimental data on Class II (cytosolic) sHsps, especially in maize. Here we report the expression and purification of two Class II sHsps from Zea mays ssp. mays L. (cv. Oh43). The two proteins have almost identical sequences, with the significant exception of an additional nine-amino-acid intervening sequence near the beginning of the N-terminus in one of them. Both ZmHsp17.0-CII and ZmHsp17.8-CII oligomerize to form dodecamers at temperatures below heat shock, and we were able to visualize these dodecamers with TEM. There are significant differences between the two sHsps during heat shock at 43°C: ZmHsp17.8-CII dissociates into smaller oligomers than ZmHsp17.0-CII, and ZmHsp17.8-CII is a more efficient chaperone with target protein citrate synthase. Together with the previous observation that ZmHsp17.0-CII but not ZmHsp17.8-CII is expressed during development, we propose different roles in the cell for these two sHsps.