The calcium binding protein calreticulin in pollen of Liriodendron tulipifera L.

We have analysed the distribution of the endoplasmic reticulum (ER) and the occurrence of the high capacity, low affinity Ca2+ binding protein calreticulin in mature pollen of Liriodendron tulipifera L. Two protein bands of 56 and 54 kDa were highlighted by antibodies against spinach calreticulin and the ER retention signal HDEL. The same proteins stained blue with Stains All on SDS-PAGE, bound Ca-45 in overlays and showed a highly acidic isoelectric point. N-terminus sequencing confirmed their identity as calreticulin isoforms, only one of which was found to be glycosylated. The role of the ER and of its resident protein calreticulin in Ca2+ signalling during pollen germination is discussed.