One of the world’s largest marketing and communications firms has acquired a stake in Just Marketing International, a locally based motorsports marketing firm that has been among central Indiana’s fastest-growing companies since its founding in 1995.

Just Marketing founder and CEO Zak Brown said Monday that London-based WPP Group is taking a minority interest in his firm. Sources close to the deal said WPP will own about 25 percent of the company.

Joint news conferences were planned in Indianapolis and London on Monday.

Brown declined to divulge financial details of the agreement, but he said Just Marketing has been valued at $100 million.

“This is huge for us,” he said, and not just because of the cash infusion. “To have a company of that stature say they believe in JMI and motorsports is a really powerful statement for us and our industry.”

Most of the money from the deal will go toward paying current investors, Brown said.

“We are strong believers in the marketing power of international motorsports for global brands,” Andrew Scott, WPP's director of corporate development, said in a statement. “JMI is a leading company in the sector, so this investment is a natural fit for WPP and will help to offer our clients exciting new opportunities in the field of motorsports.”

Zionsville-based Just Marketing will have access to WPP’s global network while providing a direct route for WPP clients to be introduced to motorsports, Scott said.

“The No. 1 most important aspect of this deal for us is, we get to introduce WPP’s clients to the world of motorsports and we get to serve them in that area,” Brown said.

Last year, WPP reported revenue of $14.8 billion. The company has 146,000 employees in 2,400 offices. WPP owns a number of the world’s best known and most powerful advertising, public relations and market research networks, including Grey, Burson-Marsteller, Hill & Knowlton, JWT, Ogilvy Group, TNS and Young & Rubicam.

According to company officials, WPP handles $70 billion in advertising annually and does business with 340 of the companies on the Fortune 500 list.

Paul Knapp, CEO of Indianapolis-based ad agency Young & Laramore, said WPP and its affiliated companies are some of the biggest and most widely known in the global advertising industry.

Knapp isn’t shocked they’d swoop in to buy a piece of a local agency.

“Giant holding companies like WPP are publicly held and have their own pressures,” Knapp said. “They need to continue to grow.”

There’s a simple reason why WPP would acquire a firm like Just Marketing, Knapp said.

“They see a potential for this company to be profitable and for them to take some of that profit,” Knapp said. “They’re in the business of acquiring companies that are doing well and making money.”

New York-based Spire Capital Partners, which took a majority stake in JMI in 2008, remains the principal shareholder and Brown the second-largest shareholder. Sources close to the company said Spire has a stake near 40 percent, while Brown owns 27 percent of the company.

In addition to its equity stake, WPP has one representative on the six-person JMI board of directors.

Brown, 39, isn’t worried about losing control of Just Marketing. And he made it clear that he is having no thoughts of retiring.

“I’ve not had control in theory since 2008,” Brown said. “That was a concern when I did the Spire deal, not because of who Spire is, but because it was the first time I gave up majority control of the company I founded.

“I think what you’ll find with private-equity firms is, they don’t want control of the company. What they want is to back existing management teams because they believe in them and they want to help contribute to their growth. They are investing in the people here, so that’s not a fear of mine. I welcome the investors pushing me.”

Just Marketing represents corporate interests that are interested in motorsports sponsorships and partnerships. The local firm’s client list consists of some of the biggest sponsors in Formula One and NASCAR, including DirecTV, Castrol, UPS, Verizon, Jeremiah Weed, Crown Royal, LG, Subway, UBS and myriad others.

Just Marketing reported revenue of $57 million in 2008, $78 million in 2009, and $76 million in 2010.

Brown’s company has become so successful in Europe that several sources have pegged him as one of the 15 most powerful people in Formula One, the world’s biggest racing circuit.

Click here for IBJ's "Leading Questions" multimedia feature on Zac Brown from last summer.