Abstract
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There is a continuous demand to improve our
understanding of fundamental
processes that underlie human health and disease. Therefore, novel
strategies that can assist in these efforts are required. For example,
molecular biology and genetic approaches have revolutionized our understanding
of protein-mediated processes by facilitating their direct visualization
and analyses in living cells. Despite these developments, genetic
manipulation has limitations in controlling events that occur after
translation such as posttranslational modifications (PTMs), which
are imperative regulatory elements. As a result, developing new methods
to study PTMs in live cells is a major bottleneck in deciphering their
exact roles in the myriad cellular processes.
Synthetic and
semisynthetic proteins are prepared by combining
solid phase peptide synthesis (SPPS) and chemoselective ligation approaches
with synthetic or recombinant peptides. Employing protein synthesis
allows chemists to incorporate natural and unnatural modifications
with virtually unlimited number of functional groups into the protein’s
sequence, such as PTMs and their mimics. In addition, synthetic proteins
can include additional elements such as fluorescent tags, reactive
groups, caged units, and enrichment handles. Therefore, harnessing
the power of chemical protein synthesis offers great opportunities
to study fundamental biological processes.
Unfortunately, the
low cell permeability of proteins limits their
applications mainly to in vitro settings, excluding
live cell studies. As a result, chemical biologists have been attempting
to overcome these limitations by developing protein delivery methods
that would enable the study of custom-made proteins in a biological
context. Success with these strategies should enable accurate determination
of protein localization, degradation, folding, interactions, and involvement
in the assembly of membrane-less organelles formed by liquid–liquid
phase separation inside cells. Importantly, protein delivery approaches
are complementary to genetic manipulations, and combining these approaches
should pave the way to new discoveries.
In this Account, we
describe recent developments in protein delivery
methods, with emphasis on those most compatible with synthetic proteins.
We highlight experimental approaches and conceptual adaptations required
to design and study synthetic proteins in live cells, with or without
genetic manipulation. In addition, we highlight the strength and weakness
of these approaches for both the delivery and the subsequent studies.
We also describe our endeavors to deliver synthetic proteins to cells
via cell penetrating peptides (CPPs) and multiplexed bead loading
(MBL), as showcases of the applications of these methods to shed light
on biological processes. Lastly, we contemplate other future applications
of synthetic proteins to answer questions that are currently unapproachable.
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