Biocompatible hydrogels have a wide variety of potential applications in biotechnology and medicine, such as the controlled delivery and release of cells, cosmetics and drugs; and as supports for cell growth and tissue engineering1. Rational peptide design and engineering are emerging as promising new routes to such functional biomaterials2-4. Here we present the first examples of rationally designed and fully characterized self-assembling hydrogels based on standard linear peptides with purely α-helical structures, which we call hydrogelating self-assembling fibres (hSAFs). These form spanning networks of α-helical fibrils that interact to give self-supporting physical hydrogels of >99% water content. The peptide sequences can be engineered to alter the underlying mechanism of gelation and, consequently, the hydrogel properties. Interestingly, for example, those with hydrogen-bonded networks melt upon heating, whereas those formed via hydrophobic interactions strengthen when warmed. The hSAFs are dual-peptide systems that only gel on mixing, which gives tight control over assembly5. These properties raise possibilities for using the hSAFs as substrates in cell culture. We have tested this in comparison with the widely used Matrigel substrate, and demonstrate that, like Matrigel, hSAFs support both growth and differentiation of rat adrenal pheochromocytoma cells for sustained periods in culture.
β‐catenin is a key protein in cadherin–catenin cell adhesion complex and its tyrosine phosphorylation is believed to cause destruction of junctional apparatus. The broad spectrum of substrates for kinases and phosphatases, however, does not rule out tyrosine phosphorylation of other junctional proteins as the main culprit in reduction of cell adhesion activity. Further, the endogenous β‐catenin perturbs detailed functional analysis of phosphorylated mutant β‐catenin in living cells. To directly evaluate the effect of β‐catenin tyrosine phosphorylation in cell adhesion, we utilized F9 cells in which expression of endogenous β‐catenin and its closely related protein plakoglobin were completely shut down. We also used α‐catenin‐deficient (αD) cells to evaluate the role of α‐catenin on β‐catenin tyrosine phosphorylation. We show that β‐catenin with phosphorylation mutation at 654th tyrosine forms functional cadherin–catenin complex to mediate strong cadherin‐mediated cell adhesion. Moreover, we show that 64th and 86th tyrosines are mainly phosphorylated in F9 cells, especially in the absence of α‐catenin. Phosphorylation of these tyrosine residues, however, does not affect cadherin‐mediated cell adhesion activity. Our data identified a novel site phosphorylated by endogenous tyrosine kinases in β‐catenin. We also demonstrate that tyrosine phosphorylation of β‐catenin might regulate cadherin‐mediated cell adhesion in a more complicated way than previously expected.
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