18-kDa hsp localization to membraneous whorl structures
--Denison, SJ; Smith, RJ; Walden, DB

Most organs of maize seedlings respond to brief heat-shock exposure with the synthesis of a variety of heat-shock proteins (hsps), particularly a group of 18-kDa hsps referred to as the small hsps (shsp). To date there is little known about the cellular 'chaperone' function of the shsps. The cytological localization of individual hsps may suggest possible functional roles and also exclude certain functions.

cDNAs from three members of the maize 18-kDa gene family have been sequenced and characterized (Atkinson et al., Genome 31:698-704, 1989; Goping et al., Plant Mol. Biol. 16:699-711, 1991) and stage specific constitutive expression of two of these has been detected during microsporogenesis (Atkinson et al., Dev. Genet. 14:15-26., 1993; Bouchard et al., Maydica 38:135-144, 1993). See also contribution from Wooster, Ohio, this volume.

We present observations made on the localization of the 18-kDa to unique membrane containing structures in heat-shocked root-tips of inbred Oh43, utilizing immunogold labelling of the 18-kDa hsp-antibody complex.

These membraneous structures, which are poorly represented in the literature, are found close to cell walls (Figure 1). These structures appear to be comprised of numerous membranes with a whorl-type conformation. These structures may be involved in 'autophagic' digestion (Gunning and Steer, Ultrastructure and the Biology of Plant Cells, pp. 183-280, 1975), or may be the result of fixation.

With chemically fixed tissue one compromises structure to retain antigenicity and vice-versa. The presence of gold-label in these membraneous whorl structures indicates that the membranes retain their antigenicity. If, in fact, fixation resulted in poor structure maintenance then these membranes are probably components of the plasma membrane based on their association with cell walls. However, it should be noted that other organelle membranes in the vicinity of these structures are undamaged, which suggests that these whorls are not fixation artifacts. These structures are not found in non-heat-shocked tissues.

Under heat-stress conditions, tissues and proteins may be damaged and therefore no longer useful. These structures, if involved in 'autophagic' digestion, may aid in the degradation of these materials. The hsp 18, described as a 'chaperone' molecule, may assist this process by the transportation of damaged material to these structures for the purposes of degradation.

Figure 1. Electron micrograph of a membraneous whorl structure in Zea mays L. heat-shocked radicle (x75,000). Note the increase in gold label to this structure relative to other components of the cell. Cell wall (CW), membraneous whorl structure (MW).

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