Plant Science 131 (1998) 35–42 The calcium binding protein calreticulin in pollen of Liriodendron tulipifera L. Lorella Navazio a , Lorena Sponga a , Paola Dainese b , Anne-Catherine Fitchette-Laine ´ c , Loı ¨c Faye c , Barbara Baldan a , Paola Mariani a, * a Dipartimento di Biologia, Uniersita ` di Padoa, Via U. Bassi 58 /B, 35131 Padoa, Italy b Institut fu ¨r Biochemie III, Eldgeno ¨ssische Technische Hochschule Zentrum, Uniersita ¨tstrasse 16, CH-8092 Zu ¨rich, Switzerland c Laboratoire des Transports Intracellulaires, CNRS -URA 203, IFRMP 23, Uniersite ´ de Rouen, 76821 Mont -Saint Aignan ce ´dex, France Received 22 September 1997; accepted 15 October 1997 Abstract We have analysed the distribution of the endoplasmic reticulum (ER) and the occurrence of the high capacity, low affinity Ca 2 + 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 45 Ca 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 Ca 2 + signalling during pollen germination is discussed. © 1998 Elsevier Science Ireland Ltd. Keywords: Calcium signalling; Calreticulin; Endoplasmic reticulum; Liriodendron tulipifera ; Pollen 1. Introduction Ca 2 + acts as a second messenger involved in a variety of physiological processes by means of strictly controlled changes in its intracellular lev- els. Modulation of cytosolic free Ca 2 + is obtained through a complex of Ca 2 + homeostatic mecha- nisms, including pumps, ion channels and Ca 2 + buffers within storage compartments [1,2]. Calreticulin (CR) is a low-affinity, high-capac- ity Ca 2 + binding protein known to play a major role in luminal buffering of the ion within the endoplasmic reticulum (ER) of animal cells [3]. Recent evidence suggests additional important functions of the protein in control of gene expres- sion [4], intraluminal Ca 2 + sensing [5], cell to cell Abbreiations: CR, calreticulin; ER, endoplasmic reticulum. * Corresponding author. Tel.: +39 49 8276277; fax: +39 49 8276280; e-mail: marianip@civ.bio.unipd.it 0168-9452/98/$19.00 © 1998 Elsevier Science Ireland Ltd. All rights reserved. PII S0168-9452(97)00233-1