Many different proteins have helicase function, not just one. Helicases are a large family of motor proteins found in every living organism and in viruses, all sharing the ability to separate double-stranded DNA or RNA into single strands. In humans alone, more than 90 helicase proteins have been identified, each specializing in different cellular tasks like DNA replication, DNA repair, gene expression, and protein production.
What Makes a Protein a Helicase
A helicase is any protein that uses the energy from breaking down ATP (the cell’s energy currency) to pry apart the two strands of a DNA or RNA double helix. At their core, most helicases share a conserved structural unit built from two RecA-like domains, named after a bacterial DNA repair protein. These two domains open and close relative to each other, creating a mechanical cycle that grips a nucleic acid strand, pulls it forward, and peels apart base pairs one at a time.
The protein advances along one strand using what researchers describe as a “wrench-and-inchworm” mechanism. ATP binding causes the two domains to clamp together, unwinding one base pair. Then, when the spent energy molecule is released, the protein shifts its grip forward along the single strand. Each ATP consumed translates to exactly one base pair separated and one nucleotide of forward movement.
All helicases contain signature amino acid sequences, particularly the Walker A motif (which forms a loop that grips the phosphate portion of ATP) and the Walker B motif (which activates the water molecule needed to break ATP apart). These motifs are the defining fingerprint that identifies a protein as a helicase. Based on variations in these sequences, helicases are divided into six superfamilies, SF1 through SF6.
Major DNA Helicases
The MCM Complex in DNA Replication
The most essential DNA helicase in human cells is the MCM2-7 complex, a ring-shaped assembly of six related proteins that belongs to superfamily 6. This is the helicase that unwinds DNA at replication forks, allowing your cells to copy their entire genome before dividing. It travels along DNA in the 3′-to-5′ direction, and without it, DNA replication cannot begin.
RecQ Helicases: Guardians of the Genome
Humans have five RecQ family helicases, often called “guardians of the genome” because they maintain DNA stability. All five travel in the 3′-to-5′ direction and belong to superfamily 2. Each has a specialized role:
- BLM suppresses abnormal DNA recombination at stalled replication forks. It works within a protein complex called the BLM dissolvasome to resolve tangled DNA intermediates without creating unwanted crossovers between chromosomes.
- WRN is unique among the five because it has both helicase and exonuclease activity, meaning it can unwind DNA and also chew back one strand.
- RECQL1 plays a role in firing replication origins (the sites where DNA copying begins) and restarting stalled replication forks.
- RECQL4 interacts directly with the MCM replication machinery, helping coordinate the start of DNA synthesis.
- RECQL5 helps regulate recombination and transcription.
Repair and Unwinding Helicases
Beyond replication, several helicases are dedicated to DNA repair. XPD (superfamily 2) travels 5′-to-3′ and is critical for a repair process that fixes bulky DNA damage caused by UV light and certain chemicals. PcrA and UvrD, well-studied bacterial helicases in superfamily 1, use the inchworm mechanism to unwind damaged DNA sections so repair enzymes can access them. The RecBCD complex in bacteria is notable for containing subunits that move in opposite directions on each strand simultaneously, rapidly processing broken DNA ends.
Major RNA Helicases
RNA helicases are even more numerous than DNA helicases in human cells. The two largest families are the DEAD-box and DEAH-box helicases, both named after the amino acid sequence in their Walker B motif. They share the same two-domain RecA core but specialize in reshaping RNA structures rather than unwinding long stretches of double-stranded nucleic acid.
The most important RNA helicase in translation (protein production) is eIF4A. It works as part of a complex that clears secondary structures from the beginning of messenger RNA molecules, allowing the ribosome to attach and begin reading the genetic code. Two versions, eIF4A1 and eIF4A2, perform this unwinding role, while a third version, eIF4A3, does not participate in translation at all and instead functions in RNA processing after splicing.
DDX3X resolves more complex RNA structures than eIF4A can handle and also plays roles in shuttling RNA between the nucleus and cytoplasm, in viral infection, and even in chromosome segregation during cell division. DHX29 sits at the entrance of the ribosome’s mRNA channel and ensures that RNA is fed through in a straight, linear fashion during scanning. DHX36 and DHX9 specialize in resolving G-quadruplexes, unusual four-stranded RNA structures that can form in the untranslated regions of certain messages and block translation if left intact.
DDX6 plays the opposite role from most translation-associated helicases: it represses protein production by helping silence specific mRNAs targeted by small regulatory RNA molecules.
Helicase Directionality
Every helicase moves along its nucleic acid track in one of two directions. Type A helicases travel 3′-to-5′, and Type B helicases travel 5′-to-3′. This polarity is fixed by the protein’s structure and determines which strand it grips during unwinding.
Most human DNA replication and repair helicases are Type A (3′-to-5′), including the MCM complex, all five RecQ helicases, and the Rep and PcrA bacterial helicases. Type B (5′-to-3′) helicases include XPD, the Pif1 family, and the bacterial DnaB replicative helicase. A few helicases, including eIF4A and DHX36, can work in both directions depending on context.
Viral Helicases
Viruses encode their own helicases to replicate their genomes inside host cells. The hepatitis C virus (HCV) NS3 protein is the best-studied example. It combines helicase and protease functions in a single protein, unwinding RNA while also cleaving the viral polyprotein into functional pieces. Its helicase portion belongs to superfamily 2, but it contains a third structural domain not found in related human helicases, making it structurally distinct.
Herpes simplex virus uses a three-protein helicase-primase complex (UL5, UL8, and UL52) to unwind its DNA genome during replication. These structural differences between viral and human helicases are significant because they create potential targets for antiviral drugs. However, because many viral SF2 helicases are closely related to essential human proteins, designing drugs that block the viral version without disrupting the human one remains challenging.
Diseases Caused by Helicase Mutations
When helicase genes carry mutations, the consequences can be severe. Three well-characterized genetic disorders result from mutations in human RecQ helicases. Bloom syndrome, caused by mutations in BLM, leads to short stature, sun sensitivity, and a dramatically elevated risk of cancer due to unchecked chromosome rearrangements. Werner syndrome, caused by WRN mutations, produces features that resemble premature aging, including early graying, cataracts, and age-related diseases appearing decades earlier than normal. Rothmund-Thomson syndrome, caused by RECQL4 mutations, involves skin abnormalities, skeletal defects, and an increased risk of bone cancer.
All three conditions reflect what happens when the genome loses a key surveillance protein: DNA damage accumulates, chromosomes become unstable, and cells either die prematurely or acquire dangerous mutations.