Aldolase Exists in Both the Fluid and Solid Phases of Cytoplasm

We have prepared a functional fluorescent analogue of the glycolytic enzyme aldolase (rhodamine [Rh]-aldolase), using the succinimidyl ester of carboxytetramethyl-rhodamine. Fluorescence redistribution after photobleaching measurements of the diffusion coefficient of Rh-aldolase in aqueous solutions gave a value of 4.7 × 10 -7 cm2/s, and no immobile fraction. In the presence of filamentous actin, there was a 4.5-fold reduction in diffusion coefficient, as well as a 36% immobile fraction, demonstrating binding of Rh-aldolase to actin. However, in the presence of a 100-fold molar excess of its substrate, fructose 1,6-diphosphate, both the mobile fraction and diffusion coefficient of Rh-aldolase returned to control levels, indicating competition between substrate binding and actin cross-linking. When Rh-aldolase was microinjected into Swiss 3T3 cells, a relatively uniform intracellular distribution of fluorescence was observed. However, there were significant spatial differences in the in vivo diffusion coefficient and mobile fraction of Rh-aldolase measured with fluorescence redistribution after photobleaching. In the perinuclear region, we measured an apparent cytoplasmic diffusion coefficient of 1.1 × 10 -7 cm2/s with a 23% immobile fraction; while measurements in the cell periphery gave a value of 5.7 × 10 -g cm2/s, with no immobile fraction. Ratio imaging of Rh-aldolase and FITCdextran indicated that FITC-dextran was relatively excluded from stress fiber domains. We interpret these data as evidence for the partitioning of aldolase between a soluble fraction in the fluid phase and a fraction associated with the solid phase of cytoplasm. The partitioning of aldolase and other glycolytic enzymes between the fluid and solid phases of cytoplasm could play a fundamental role in the control of glycolysis, the organization of cytoplasm, and cell motility. The concepts and experimental approaches described in this study can be applied to other cellular biochemical processes. TIaOUGH glycolytic metabolism is well-defined at the biochemical level, the dynamic and spatial organization of glycolysis in living cells has not been characterized to date. After the elucidation of mitochondrial structure and function 35 years ago (28) it was generally assumed that a corresponding organelle for glycolysis must exist. Efforts to isolate and to characterize such an organelle were unsuccessful, leading de Duve (26) to conclude that the longsought"glycolytic particle" did not exist. As a result, the current concept of in vivo glycolytic metabolism is based on an assumption of dilute solution chemistry, due largely to the absence of evidence for a discrete glycolytic organelle. There has been evidence for over 20 years that some glycoiytic enzymes bind to structural proteins, particularly actin. The interaction of aldolase with actin has been demonstrated histochemically (8, 61), biochemically (5, 6, 8, 69), ultrastructurally (16, 52), and with immunofluorescence techniques (27). In fact, aldolase was among the first actinbinding proteins to be identified (10), although this interaction was not considered specific at the time. It has since been well-characterized in terms of both its native structure, enzymatic activity, and its actin-binding qualities. Some of the characteristics of vertebrate striated muscle aldolase are reviewed in Table I. Aldolases from other mammalian sources are reasonably well-conserved (56). The potential significance of the localization of glycolytic enzymes has been indicated by reconstitution experiments under conditions of physiological ionic strength and protein concentration (15), and in the presence of the regulatory proteins of skeletal muscle (75). This work suggested that previously reported "glycolytic component" complexes (13, 14, 34) were actually complexes of glycolytic enzymes with structural proteins, and implied that the glycolytic particle might exist as such a complex. The model for the cytoplasmic organization of glycolytic enzymes that has emerged from Clarke and Masters' work is that there is a "plating" of glycolytic enzymes around the filamentous actin (F-actin) ~ core of microfilaments (48). Masters proposed a dynamic t. Abbreviations used in this paper: Daq, aqueous diffusion coefficient; Dcyto, apparent cytoplasmic diffusion coefficient; D/P, dye/protein ratio; F-actin, filamentous actin; FDP, fructose 1,6-diphosphate; FRAP, fluorescence redistribution after photobleaching; Rh, rhodamine. © The Rockefeller University Press, 0021-9525/88/09/981/11 $2.00 The Journal of Cell Biology, Volume 107, September 1988 981-991 981 on A ril 7, 2008 w w w .jc.org D ow nladed fom Table I. Properties of Vertebrate Striated Muscle Aldolase Property Reference No.