* Futures down: Dow 13 pts, S&P 3 pts, Nasdaq 8.5 pts
By Sweta Singh
May 29 (Reuters) - U.S. stock index futures were lower on Friday amid worries about Greece's ability to strike a deal with its euro zone partners by Sunday and ahead of U.S. gross domestic product (GDP) data.
* A euro zone official said Greece will not be able to get the money still available under its current bailout plan if it does not agree to the outline of a such a deal by the end of the week.
* The Commerce Department is expected to report U.S. economy likely shrank in the first quarter instead of growing, as was estimated last month. The data is due at 8:30 a.m. ET (1230 GMT).
* While several economists have cast doubts on the accuracy of GDP estimates for the quarter, investors will peruse the data for clues to narrow down the timing of an increase in interest rates.
* With growth estimates so far for the second quarter around 2 percent, the economy appears poised for its worst first half performance since 2011.
* The GDP report is also expected to show a second quarterly drop in corporate profits because of the strong dollar and oil prices.
* Altera rose 6.2 percent to $49.90 in premarket trading after the New York Post reported that Intel was close to buying the chipmaker for about $15 billion. Intel was up 0.85 percent to $34.30.
* Splunk fell 1.1 percent to $70.30 after the data analytics software maker posted a bigger quarterly loss, hurt by higher expenses.
* Heron Therapeutics rose 35.63 percent to $16.75 after the company's nausea drug met the main goal in a late-stage trial.
Futures snapshot at 7:34 a.m. ET (1134 GMT):
* S&P 500 e-minis were down 3 points, or 0.14 percent, with 118,694 contracts traded.
* Nasdaq 100 e-minis were down 8.5 points, or 0.19 percent, on volume of 19,130 contracts.
* Dow e-minis were down 13 points, or 0.07 percent, with 21,835 contracts changing hands. (Additional reporting by Sayantani Ghosh in Bengaluru; Editing by Savio D'Souza)