Bead rings at the endoplasmic reticulum-Golgi complex boundary: morphological changes accompanying inhibition of intracellular transport of secretory proteins in arthropod fat body tissue

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

Golgi complex beads and the transition region

Secretory proteins and membranes move in transfer vesicles from the rough endoplasmic reticulum through the transition region to the outer saccule of the Golgi complex. In both arthropod and vertebrate cells, the GC beads are a characteristic structural component of the transitional region. The beads are particles about half the size of ribosomes arranged equidistantly from one another and the smooth face of the ER. In an active GC, the beads are in rings through which the ER membrane emerges to form transfer vesicles. The beads may be part of the energy-dependent step required for the movement of proteins along eht secretory pathway, since they lose their ring arrangement under conditions that lower cellular ATP. The beads are organizers for Golgi complexes in the sense that they are the first recognizable components of new GCs as they arise from ER. Arthropod GC beads, but not those of vertebrates, can be visualized through their reaction with bismuth in vivo and in fixed tissue. Useful paradigms for traffic between the ER and the GC need to combine structural and biochemical information. Insect fat body, with its readily resolvable bismuth-strained beads and easily fractionated cell components may have particular value for this problem.

The charge distribution in the rough endoplasmic reticulum/Golgi complex transitional area investigated by microinjection of charged tracers

The distribution of microinjected ferritin, ranging in charge from anionic to highly cationic, has been used to indicate differences in surface charge on the rough endoplasmic reticulum and the Golgi complex of intact cells. Highly cationic ferritins (HCF)(HCF1, pI 7.9-9.1; HCF2, pI 8.5-9.4; and HCF3.pI 9.5-10.1) were mostly bound and caused swelling of the rough endoplasmic reticulum. Cationic ferritin (CF) (pI 7.0-8.0) and anionic ferritin (AF) (pI 4.0-4.4) caused no changes in morphology. The distribution of these ferritins in the cytoplasmic space varied with their charge. Significantly more CF was bound to surfaces than was found in the free cytoplasmic space. Conversely, there was significantly more AF in the free cytoplasmic space than close to surfaces. Therefore, the intracellular surfaces are negatively charged. Comparison of the structures in the secretory pathway showed no differences in ferritin binding to transition vesicles, rough endoplasmic reticulum, Golgi saccules or secretory vesicles. The Golgi complex beads are not distinguished by their charge. It is therefore unlikely that charge differences play a role in regulating membrane-membrane interactions in this region of the secretory pathway.

Lead staining in the Golgi complex

Lead ions at similar concentrations to those used for Gomori type phosphatase localization stain some parts of the vacuolar system, particularly compartments of the Golgi complex (GC) and isolation envelopes (im) in a characteristic way in both vertebrates and invertebrates. After fixation in 2.5% glutaraldehyde, lead citrate in acetate or aspartate buffer (pH 5.5-7.2) leaves the contents of GC cisternal compartments with a fine particulate stippling. In the fat body of Calpodes ethlius and in mouse pancreas the staining is faint but definite without further enhancement of contrast, although it is easily overlooked after section staining. The distribution of lead stain differs from that of the lead phosphate precipitated after Gomori type acid phosphatase reactions. Whereas lead stain may be in all GC and im compartments, acid phosphatase is restricted to the innermost saccules and nearby vacuoles. The compartment specific staining by led also differs from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC from the generalized staining in all compartments given by uranyl. Thus the contents of luminal membrane surfaces of some parts of the vacuolar system can be characterized by their ability to bind lead. In cells where protein synthesis has been blocked by cycloheximide, secretory vesicles are absent and the RER and GC cisternae are devoid of uranyl stainable material. However, lead staining and acid phosphatase activity in the GC continue. We presume that they mark the environment within these cisternae rather than the proteins passing through them. This environment is itself not static. Several observations suggest that the function of cisternae that is detectable by lead staining is temporally discontinuous and related to a stage of maturation or development. Only early stage ims stain: the staining ceases by the beginning of autophagy after hydrolytic enzymes are presumed to have been added. Condensing vacuoles cease to stain as the central core crystallizes out. Stain may be absent from one or two GC saccules at any position in the stack as though the phase of lead staining (or lack or it) can move progressively through the system. We conclude that in studies characterizing components of the vacuolar system it is necessary to separate those that mark transient occupants of a compartment from those that mark the compartment itself. Both may vary temporally independently from one another.

The arrangement of the Golgi complex beads is not controlled by the cytoskeleton

Exposure of insect fat body to treatments which disrupt microtubules (colchicine, vinblastine sulfate and cold treatment) blocks intracellular transport between the Golgi complex and the plasma membrane but does not affect Golgi complex bead rings or transport from rough endoplasmic reticulum to the Golgi complex. Drugs which disrupt microfilaments (cytochalasins B and D) do not affect the bead rings or intracellular transport of secretory proteins at any level. Thus, intracellular transport between the rough endoplasmic reticulum and the Golgi complex and the arrangement of the beads in rings are both independent of the cytoskeleton. The ring arrangement is presumably maintained by interconnection(s) with rough endoplasmic reticulum membrane.

High resolution microanalysis for phosphorus in Golgi complex beads of insect fat body tissue by electron spectroscopic imaging

Golgi complex beads are 10 nm particles arranged in rings on the smooth forming face of the Golgi complex that stain specifically with bismuth in arthropod cells. In vitro experiments with biological molecules spotted on to cellulose acetate strips indicated that bismuth bound to the beads through phosphate groups. We could detect a weak phosphorus signal from the beads using a new technique called electron spectroscopic imaging that is capable of very high spatial resolution (0.3-0.5 nm) and sensitivity (50 atoms of phosphorus). Detection was not obscured by tissue staining with bismuth or uranyl acetate of by using an inorganic buffer (Na cacodylate). Localization of phosphorus was greatly improved by using colour-enhanced computer pictures of the electron spectroscopic images and quantitating the images. The results indicate that the phosphorus content of the beads is large enough to account for their bismuth reactivity.