A proline-rich loop mediates specific functions of human sialidase NEU4 in SK-N-BE neuronal differentiation

Human sialidase NEU4 long is a membrane-associated enzyme that has been shown to be localized in the outer mitochondrial membrane. A role in different cellular processes has been suggested for this enzyme, such as apoptosis, neuronal differentiation and tumorigenesis. However, the molecular bases for these roles, not found in any of the other highly similar human sialidases, are not understood. We have found that a proline-rich sequence of 81 amino acids, unique to NEU4 sequence, contains potential Akt and Erk1 kinase motifs. Molecular modelling, based on the experimentally determined 3D structure of cytosolic human NEU2, showed that the proline-rich sequence is accommodated in a loop, thus preserving the typical beta-barrel structure of sialidases. In order to investigate the role of this loop in neuronal differentiation, we obtained SK-N-BE neuroblastoma cells stably overexpressing either human wild type NEU4 long or a deletion mutant lacking the proline-rich loop. Our results demonstrate that the proline-rich region can also enhance cell proliferation and retinoic acid induced neuronal differentiation and it is also involved in NEU4 interaction with Akt, as well as in substrate recognition, modifying directly or through the interaction with other protein(s) the enzyme specificity toward sialylated glycoprotein(s). On the whole, our results suggest that NEU4 long could be a downstream component of PI3K/Akt signalling pathway required for retinoic acid induced differentiation of neuroblastoma SK-N-BE cells.