Solubility, protein disulfide-isomerase

In ABL, an early step in apoB lipoprotein assembly shared by intestinal and liver cells is defective. The net result is near absence of all plasma apoB lipoproteins. ApoB synthesis from a mRNA transcript occurs, but its successful assembly into the mature lipoprotein particle does not. The inability to assemble apoB into lipoproteins was shown to be due to a defect in the mttp gene in affected individuals (Wetterau et al., 1992). Its translational product is an 894-amino acid, 97-kd, polypeptide that exists in the ER complexed with a 55-kd protein disulfide isomerase which is believed to maintain solubility, physiologic activity, and ER retention of the 97-kd peptide. The heterodimeric complex of the 97-kd and 55-kd subunits is referred to as microsomal triglyceride transfer protein (MTP) (Wetterau et al., 1992). 

As discussed in Chapter 16, the ER contains several soluble proteins dedicated to the folding and modification of newly synthesized secretory proteins. These Include the chaperone BiP and the enzyme protein disulfide Isomerase, which are necessary for the ER to carry out Its functions. Although such ER-resident luminal proteins are not specifically selected by COPII vesicles, their sheer abundance causes them to be continuously loaded passively into vesicles destined for the cis-Golgi. The transport of these soluble proteins back to the ER, mediated by COPI vesicles, prevents their eventual depletion... 

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