Search

All cells can be assigned to one of two categories based on the complexity of cellular organization, eukaryotes and prokaryotes. Eukaryotes possess, among other distinguishing features, an intracellular dynamic membrane system through which there is a constant flow of membranes scaffolded by an internal cytoskeleton. Prokaryotes, however, can have internal membranes, entirely lack a system that resembles eukaryotic endomembranes in terms of dynamics, complexity and the multitude of functions. How and why did the complex endomembrane system of eukaryotes arise? Here I give a critical overview of the different cell biological model scenarios that have been proposed to explain endomembrane origins. I argue that the widely held symbiotic models for the origin of the nuclear envelope and other endomembranes are cell biologically and evolutionarily highly implausible. Recent findings about the origin of nuclear pore complexes also severely challenge such models. I also criticize a scenario of de novo vesicle formation at the origin of the endomembrane system. I contrast these scenarios to traditional and revised autogenous models according to which eukaryotic endomembranes evolved by the inward budding of a prokaryotic cell΄s plasma membrane. I argue that such models can best satisfy the major constraints of membrane topology, membrane heredity and straightforwardly account for selection pressures while being consistent with genomic findings. [ABSTRACT FROM AUTHOR]

As reflected in this monograph, a lot of knowledge about the lysosomal compartment has been acquired since the discovery of the organelle by de Duve and others in the 1950s. Nevertheless, a lot of questions are still open, and some of them can be addressed using screening techniques. Among them are questions about the structure of the lysosomal mem-brane, the function of its proteins, and the effect of lysosomal dysfunction on cell functions. Although the proteins of the lysosomal matrix are well known, little is realised about the proteins of the lysosomal membrane. In the first section of this chapter, we describe the most important proteome analysis related techniques used to purify and analyse lysosomal structures that can help to define the complete set of matrix proteins and to describe the pro-teins of the lysosomal membrane. In the second section, we delineate the gene expression pro-filing technique and briefly describe approaches which can be used to analyse the influence of lysosomal dysfunctions on the cell, as well as on the function of lysosomal proteins. Because transcriptome and proteome analyses of lysosomal structure and function are still only in the initial stage, we will focus on the description of the methods and the major aims. [ABSTRACT FROM AUTHOR]

The lysosomal limiting membrane has multiple functions including acidification of the lysosomal matrix, sequestration of lysosomal enzymes, mediation of fusion events beween lysosomes and other organelles, and transport of degradation products to the cytoplasm. Lysosomal membrane proteins are usually highly glycosylated proteins decorating the luminal surface of lysosomal membranes. LAMP-1, LAMP-2 and LIMP-2 are the most abundant components of this membrane. Experiments on knockout mice have demonstrated that these proteins are important for normal cell physiology and that they can be involved in pathological conditions. The deficiency of LAMP-1 causes only a mild phenotype and also no apparent lysosomal dysfunctions. It is likely that the lack of LAMP-1 may be compensated by the structurally related LAMP-2. A role for LAMP-2 in the so-called chaperone-mediated autophagy has been described. Furthermore, LAMP-2 deficiency in mice has revealed roles in lysosomal enzyme targeting, autophagy and lysosomal biogenesis. LAMP-2-deficient mice exhibit a similar phenotype to individuals with Danon disease, which is caused by mutations in the LAMP-2 gene. Experiments on LIMP-2 knockout mice and overexpression studies also suggested specific functions for this protein in maintaining normal lysosomal biogenesis. Apart from the major proteins of the lysosomal membrane, about 20 less abundant or transient components of this membrane have been described. Although it is known that mutations in some of these proteins are associated with human disease, for most of the lysosomal membrane components the actual function is yet to be determined. [ABSTRACT FROM AUTHOR]