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Previously, we described the isolation of a cDNA clone and the gene encoding a protective antigen of the protozoan parasite Entamoeba histolytica, the serine-rich Entamoeba histolytica protein (SREHP). The derived amino acid sequence of the SREHP cDNA clone was remarkable for a high serine content (52/233 amino acids), a putative signal sequence, multiple hydrophilic dodecapeptide and octapeptide tandem repeats, and a hydrophobic C-terminal putative membrane-spanning region. Here, we show that SREHP is modified by the addition of phosphate at serine residues, O-linked terminal N-acetylglucosamine residues, and by acylation. When the SREHP gene is expressed in baculovirus transformed Sf-9 cells, the product is also phosphorylated and glycosylated and is localized to the plasma membrane of the insect cells. The native SREHP molecule also serves as a potent chemoattractant for amebic trophozoites. The data presented here suggest that SREHP is a unique membrane protein with phosphorylation and glycosylation patterns usually associated with nuclear or cytoplasmic proteins.

Five types of bursting neurons have been described in Aplysia: three types of individual bursters--the LUQ cells, L10, and R15, plus two types of population bursters--the bag cells and the R25/L25 cells. Individual bursters can burst without any synaptic input, while bursts generated by the population bursters are shaped largely by their synaptic interactions. In this paper we review what is known about the burst mechanisms of these five classes of neurons and attempt to relate them to the roles of the five cell types in the control of autonomic function.

Bivalve and gastropod molluscs undergo large changes in external environmental conditions, as well as in internal state. Cardiac responses to these changing conditions have been recorded in a variety of species. There is a general tendency for heart rate, and presumably cardiac output, to increase in response to situations that would increase the load on respiratory and excretory systems. Changes in molluscan heart function in many cases appear not to be mediated directly by cardiac nerves, but rather by such indirect mechanisms as changes in blood constituents or mechanical, hemodynamic effects on heart muscle. Three types of cardiac response in Aplysia have been shown to be mediated, at least in part, by the heart regulator nerves. The neural circuits that regulate heart rate in Aplysia and in Helix have been partially described in cellular detail. In Mercenaria, Aplysia and other molluscan species there is evidence that cyclic adenosine monophosphate has a role in mediating the excitatory effects of serotonin on heart muscle. There appears to be, in fact, a general tendency in the Aplysia nervous system for neurons that exert tonic, modulatory effects within neural networks that control a variety of behaviors to use serotonin for a transmitter. In each case there is evidence to suggest that changes in cyclic adenosine monophosphate levels may mediate some of the modulatory effects of serotonin.