A sphingomyelin cycle was discovered in human myelocytic leukemia HL-60 cells which is activated during differentiation induced with 1α, 25-dihydroxyvitamin D3. In turn, ceramide, the product of sphingomyelin hydrolysis, plays an important role as a lipid messenger in cell functions including proliferation, differentiation and apoptosis. A novel magnesium-independent, neutral, cytosolic sphingomyelinase was identified as a key enzyme up-stream of ceramide-related signal transduction. The use of cell-permeable ceramides has identified protein phosphorylation and dephosphorylation and nuclear transcription factors as down-stream effectors of the ceramide signal. The implications of this novel lipid signal pathway are discussed.
The collectins are soluble lectins containing collagen-like regions which are found in plasma and in the lung surfactant. They have been characterised in mammals and birds and the group includes three plasma proteins (mannanbinding protein, bovine conglutinin and bovine collectin-43) and two lung surfactant proteins (SP-A and SP-D). They have an overall similarity in subunit, and three-dimensional structure, to each other, and also to the complement component C1q, in that they all contain globular ‘heads’ linked to triplehelical, collagen-like ‘stalks’. The globular ‘heads’ of the collectins are able to recognize carbohydrate structures on pathogens and thus trigger effector functions, or cell-binding, via the collagen-like ‘stalks’. Thus the collectins by virtue of their antibody-independent, carbohydrate recognition properties may play an important role in innate immunity especially in the very young or in those with an inability to produce antibodies.
The Golgi apparatus is one of the major organelles along the exocytotic or secretory pathway that conveys newly synthesized proteins to the cell surface. Proteins travelling through the Golgi undergo a strictly ordered series of glycosylation modifications by enzymes residing in the Golgi's different subcompartments and are sorted to their final destination when they exit the Golgi. Resident Golgi proteins must therefore be able to maintain their location in the face of heavy, surface-directed traffic. Recently, sequences mediating the retention of proteins in different subcompartments of the Golgi have been revealed and it appears that the mechanism of retention for glycosyltransferases may be different from that of a recycling protein that accumulates in the trans-Golgi network. The former relying on sequences in the transmembrane domain and is enhanced by the adjacent cytoplasmic and lumenal domains, as opposed to the latter which requires a tyrosinecontaining cytoplasmic motif that resembles the signal for basolateral targeting and rapid endocytosis.
The 35kDa glycoprotein, glycophorin A (GPA), is the most abundant sialylated glycoprotein (≈0.2-1x106 copies per cell) on human red blood cells (1). It was the first membrane protein to be fully sequenced and is still one of the best studied single pass transmembrane proteins, comprising 131 amino acids (2, 3). What has fascinated glycobiologists about GPA is its extensively glycosylated extracellular domain, and the extraordinarily high sialic acid content (4-9). There are several recent reviews on the human erythrocyte glycophorins that cover the molecular characterisation of the GPA gene, its variants (10) and the structure and function of GPA antigens (1). The purpose of this review is to summarise studies associated with the glycosylation of GPA, the sites of glycosylation, the types of sugars present on GPA and its variants, and the possible role(s) that the carbohydrates may play in the function of GPA. In addition we review the advances in technology associated with glycosylation site identification, particularly heavily O-glycosylated domains typical of GPA and mucins (11).