Apolipoprotein B: from editosome to proteasome

Apolipoprotein (apo) B, the protein component of low-density lipoproteins (LDLs), has been under intense investigation for the last three decades. During the first decade after its initial description, most reports dealt with the physical-chemical characterization of apoB in its natural environment (i.e., intact LDL particles). A few studies dealing with attempts to elucidate the primary structure of apoB were published at this time (Deutsch et al., 1978; Bradley et al., 1980). However, most of these, in retrospect, represented heroic efforts that were doomed to failure because of the huge size and insoluble nature of apoB, once it is separated from its lipid environment. Indeed, during the 1970s, there was no universal agreement on the true molecular weight of the protein, which was not established until sometime into the second decade of apoB research (Yang et al., 1986b). The next 10 years were punctuated by breakthroughs on three different fronts in our understanding of apoB. The first exciting discovery was that apoB exists in two forms, apoB-100 and apoB-48 (Kane et al., 1980; Elovson et al., 1981). The next breakthrough was the elucidation of the primary structure of apoB-100 by a combination of cDNA cloning (Chen et al., 1986; Knott et al., 1986; Yang et al., 1986a) and direct peptide sequencing (Yang et al., 1986a, 1989). This decade of renaissance in apoB research was concluded by the elucidation of the structure of apoB-48. More important in terms of basic cellular molecular biology was the discovery of RNA editing, when apoB-48 was found to be the translation product of an edited apoB mRNA (Chen et al., 1987; Powell et al., 1987). RNA editing had just been described for a kinetoplastid protozoa the year before (Benne et al., 1986). ApoB mRNA editing was the first instance of RNA editing described in a higher eukaryote (Chan and Seeburg, 1995; Grosjean and Benne. 1998). The last decade, which brings us to the present, has been marked by studies that benefited from the breakthroughs of the 1980s. which enabled many different laboratories to examine various aspects of apoB structure, function, and expression. The function of apoB in vivo was analyzed in different animal models (e.g., transgenic animals that overexpress apoB) (Linton et al., 1993; Callow and Rubin, 1995; Veniant et al., 1997) and in knockout animals that have no functional apoB (Farese et al., 1995,1996; Huang et al., 1995,1996). Furthermore, the structure-function relationship of apoB has been investigated in mice that express site-specific apoB mutants (Callow and Rubin, 1995; Veniant et al., 1997: Borén et al., 1998). A breakthrough in a related area led to the identification and cloning of microsomal triglyceride transfer protein (MTP) (Wetterau and Zilversmitt, 1984: Wetterau et al., 1992; Sharp et al., 1993) and the demonstration that MTP is essential for apoB production (Gordon et al., 1994; Leiper et al., 1994). The absence of MTP was found to lead to the complete degradation of apoB, which harks back to an observation in 1987 that, even in the presence of MTP, a substantial proportion of newly synthesized apoB-100 undergoes intracellular degradation before secretion (Borchardt and Davis, 1987). Indeed, the intracellular degradation of apoB-100 is the major determinant of its production rate from the liver, since the transcription of apoB appears to be constitutive and not subject to much regulation (Pullinger et al., 1989). It was in 1996, almost a decade after the first description of apoB's destruction inside the cell, that the proteasome-ubiquitin pathway was found to be the major mechanism for the intracellular degradation of apoB-100 (Yeung et al., 1996). Another important development within the last decade was the cloning of APOBEC-1, the catalytic subunit of the apoB mRNA editing complex (editosome) (Teng et al., 1993). This chapter will review some of the major landmarks in apoB research in the last 10 to 15 years, concentrating mainl