The Golgi apparatus of the scaly green flagellate Scherffelia dubia : uncoupling of glycoprotein and polysaccharide synthesis during flagellar regeneration

Abstract

The flagella of the green alga Scherffelia dubia are covered by scales which consist of acidic polysaccharides and glycoproteins. Experimental deflagellation results in the regeneration of flagella complete with scales. During flagellar regeneration, scales are newly synthesized in the Golgi apparatus, exocytosed and deposited on the growing flagella. Flagellar regeneration is dependent upon protein synthesis and N-glycosylation, as it is blocked by cycloheximide and partially inhibited by tunicamycin. Metabolic labeling with [35S]methionine/cysteine demonstrated that scale-associated proteins were not newly synthesized during flagellar regeneration, suggesting that the proteins deposited on regenerating flagella were drawn from a pool. Quantitative immunoelectron microscopy using a monospecific antibody directed against a scale-associated protein of 126 kDa (SAP126) revealed that the pool of SAP126 was primarily located at the plasma membrane, with minor labeling of the scale reticulum and trans-Golgi cisternae, both before deflagellation and during flagellar regeneration. Since SAP126 was sequestered during flagellar regeneration into secretory vesicles together with newly synthesized scales, it is concluded that the persistent presence of SAP126 in the trans-Golgi cisternae during scale biogenesis requires retrograde transport of the protein from the plasma membrane to the Golgi apparatus.

Models for Golgi Traffic: A Critical Assessment

“…Scaleforming algae can serve as a "reality check" for models of Golgi traffic (Becker et al 1995). Figure 6A shows a thin-section electron micrograph of S. dubia (Perasso et al 2000). Two Golgi stacks are visible, but they are separated by the nucleus, strongly suggesting that each stack operates as an independent trafficking device.…”

“…Scale bar, 1.0 mm. (Panel A adapted from Perasso et al [2000] and reprinted with permission from Springer #2000.) (B) An electron tomographic slice of a single S. dubia Golgi stack next to a transitional ER site (tER).…”

Models of Intracellular Transport: Pros and Cons

“…The data by Becker et al (1995) was presented not in the original paper but in a review. Moreover, it was then shown that this experimental model was incorrect (Perasso et al, 2000). The diameter of pre-chylomicrons is greater that the diameter of the internal volume of COPI-dependent vesicles.…”

6 Contractile vacuole s in green algae – structure and function

“…At the end of the diastolic phase, the SR is disconnected from the large round CV. Membrane structures representing collapsed CVs have not been reported in S. dubia (Melkonian and Preisig 1986;Perasso et al 2000;Becker and Hickisch 2005).…”