Scanning Transmission Electron Microscopy (STEM) Studies of Molluscan Hemocyanins

Abstract

In the past four years we have studied some 20 molluscan Hcs of the Polyplacaphoran, Gastropodan and Bivalvian classes by STEM and other physical methods (1-6). With STEM we have been able to measure the masses of individual particles in unstained, freeze-dried specimens, and to examine the arrangement of the cylindrical decameric units within various aggregates. Although the most intensively studied Hcs (e.g.,Helix pomatia) are didecameric, many of the gastropodan Hcs that we have studied are multi-decameric assemblies. The appearance of the di-decameric Hc may be represented schematically as a closed box composed of two decameric units facing one another, [x], where x is used to identify such units in the longer assemblies. In the bracket notation of Van Holde and Miller (7) for the decamer, ], the closed end represents the collar end formed by the folding over of two of the eight functional units of each of the ten monomeric chains (8). The model for a di decameric Hc is based primarily on Mellema and King’s image analyses (9) of negatively-stained transmission electron microscopic (TEM) images of the isoionic (pi) polymers of Kelletia kelletia Hc that look like stacks of closed boxes: [x][x][x]. We have found that the isoionic type of regular stacking is not seen in multi-decameric Hcs. Rather, as others have also noted (10,11), there is a polarity in the arrangement of the decameric units: usually only one “Mellema- Klug” di-decamer is present with decamers added in both directions, and with collar ends never facing one another.